After having developed myself strong arguments for idealism
with a focus on quantum physics, but also strongly criticized the
poor quality of Bernardo Kastrup's work for this purpose,
and yet not seeing the word spread much about it (well I'm not
surprised anyway as I long knew how much popularity has nothing to
do with quality), I undertook here (August 2026) to use Chatgpt to
make my case further by justifying the relevance of my work for
those who would not pay attention. Namely, to clarify how I made a
better case with more scientifically relevant concerns than
Kastrup, in particular by my focus on developing the mind makes
collapse interpretation and making a systematic review and
criticism of mainstream (physicalist) interpretations of quantum
physics, which Kastrup only occasionally sketched mainly in two
papers I had cited in the references of my big article
(as I happen to agree with these), and which got cited here by
chatgpt too, but which I did not see cited in discussions by
Kastrup's followers, who prefered to praise some less relevant
works of his instead.
In other words, the points here are:
Bernardo Kastrup is taken seriously by some philosophers of mind and consciousness researchers, but his specific “analytic idealism” is very far from being a mainstream scientific view. The distinction between Kastrup being a serious intellectual and scientists accepting his conclusions is important.
Kastrup argues that consciousness is fundamental, rather than something produced by non-conscious matter. His version of idealism says, roughly, that reality is fundamentally experiential/mental, while brains and physical objects are representations or localized appearances within that underlying consciousness.
That is a respectable position within the philosophy of mind/metaphysics. Kastrup has published philosophical papers in academic journals—for example, Disputatio and Philosophies—and his arguments engage directly with physicalism, panpsychism, the hard problem of consciousness, etc. (Reference Global)
But that doesn't mean his theory has been scientifically established.
If by "scientists" you mean working physicists, neuroscientists, biologists, etc., analytic idealism is decidedly not the consensus position.
The dominant scientific working assumption remains some form of physicalism/materialism: whatever the ultimate metaphysics turns out to be, neuroscience treats conscious experience as dependent on brain activity. Idealism doesn't currently have an established empirical research program that demonstrates that the brain is merely a representation of a more fundamental universal consciousness.
Kastrup himself acknowledges that idealism is not the mainstream position; he has explicitly complained that academia is dominated by physicalism and competing forms of panpsychism. (Bernardo Kastrup)
There is nevertheless a genuine intellectual debate here. Consciousness is an unusually difficult problem, and alternatives to straightforward physicalism—including panpsychism, cosmopsychism, idealism, and neutral monism—are discussed seriously in contemporary philosophy of mind.
This is where I would be cautious about some of Kastrup's rhetoric.
He sometimes presents scientific findings as providing substantial support for idealism. For example, he argues that idealism is empirically compatible with physics and neuroscience and has invoked research associated with quantum mechanics and Integrated Information Theory. (Bernardo Kastrup)
But “consistent with the evidence” is much weaker than “supported by the evidence.”
The same experimental observations can generally be accommodated by multiple metaphysical interpretations. Neuroscience showing correlations between brain states and conscious states, for example, doesn't by itself establish either:
“consciousness is generated by matter,” or
“matter is an appearance within consciousness.”
Those are competing interpretations of the underlying facts.
That's one reason many scientists would regard Kastrup's argument as metaphysics rather than empirical science.
There has been some substantive criticism. A particularly interesting example is Rupert Sheldrake, who has engaged directly with Kastrup's analytic idealism. In 2024, Sheldrake criticized aspects of Kastrup's position, and Kastrup published a detailed response. Their disagreement illustrates something important: even people who are sympathetic to unconventional approaches to consciousness don't necessarily accept Kastrup's particular ontology. (Bernardo Kastrup)
There is also a broader philosophical criticism: Kastrup may successfully show that physicalism isn't logically inevitable without thereby showing that idealism is true. That's probably the central issue.
I'd put it approximately like this:
| Community | Reputation of Kastrup |
|---|---|
| Mainstream physicists | Fringe/unconventional metaphysics |
| Mainstream neuroscientists | Not an accepted scientific theory |
| Philosophy of mind | Serious but highly controversial minority position |
| Consciousness researchers | Interesting and provocative, but far from consensus |
| Popular consciousness/spirituality community | Very influential |
| Idealist/panpsychist philosophers | An important contemporary advocate |
One thing that makes Kastrup unusual is that he isn't simply an internet mystic presenting a scientific-sounding theory. He has two PhDs (including one in philosophy), has a background involving CERN, and has published philosophical work in academic venues. His position therefore deserves substantially more serious engagement than the label “pseudoscience” would suggest. At the same time, his credentials don't turn analytic idealism into an empirically established scientific theory. (Bernardo Kastrup)
If someone told me:
“Kastrup has demonstrated that consciousness is fundamental and that physical reality exists within a universal consciousness.”
I'd say no—that conclusion is not accepted by science, and the evidence doesn't currently warrant saying it has been demonstrated.
If they said:
“Kastrup has developed a sophisticated philosophical argument that idealism may solve some problems that are difficult for physicalism and panpsychism.”
I'd say yes, that's a perfectly legitimate characterization, and his arguments are worth examining.
In other words, Kastrup is considerably more respectable intellectually than his popularity among some New Age audiences might suggest, but considerably less scientifically established than his strongest supporters sometimes imply.
If you'd like, I can also explain the strongest
scientific/philosophical objections to Kastrup's argument—and
whether they actually defeat analytic idealism. That
gets quite interesting, because some of the objections are much
stronger than “but neuroscience shows consciousness comes from the
brain.”
Yes. And the interesting thing is that the strongest objections aren't “science proves matter is fundamental.” They are mostly objections to whether Kastrup's idealism actually explains more than its competitors.
Kastrup's basic move is:
Instead of asking how consciousness emerges from matter, start with consciousness as fundamental and treat the physical world as the appearance of processes occurring within consciousness.
That does sidestep the classic “hard problem.” But critics argue that it creates several new problems.
This is probably the most serious objection.
Kastrup says there is ultimately one universal consciousness, while individual organisms are relatively isolated or “dissociated” portions of it.
So your consciousness and mine aren't ultimately two fundamental consciousnesses. We're two dissociated perspectives within the same underlying mind.
The obvious question is:
Why can't I experience your experiences?
If there is literally one consciousness underlying us both, why is my experience so rigorously separated from yours?
Kastrup uses dissociation as the model: something like one mind developing distinct centers of experience. But critics argue that the analogy doesn't yet constitute an explanation of how universal consciousness becomes multiple private subjects.
This is analogous to the famous combination problem in panpsychism, except Kastrup has what might be called a decombination/dissociation problem: instead of explaining how many little consciousnesses combine into one, he has to explain how one consciousness becomes many apparently autonomous ones. Kastrup specifically presents his theory as avoiding the combination problem. (Stanford Encyclopedia of Philosophy)
This is the objection I find particularly powerful.
Physicalism has the question:
How can subjective experience arise from non-conscious physical stuff?
Kastrup answers:
It doesn't. Experience is fundamental.
That's coherent—but now you have:
Why does fundamental consciousness have this particular structure?
Why does it produce electrons, galaxies, brains, evolutionary history, mathematical regularities, etc.?
And why do brains systematically alter the contents of consciousness?
So the critic can say:
You haven't explained consciousness; you've declared it fundamental and transferred the explanatory burden elsewhere.
This isn't necessarily fatal. Physics itself takes some things as fundamental. But Kastrup needs to show that his fundamental consciousness is a better theoretical primitive than fundamental physical structure.
This is the strongest scientific objection.
We know that changing the brain changes conscious experience with extraordinary reliability:
anesthesia can eliminate ordinary consciousness;
stimulation of particular brain regions can produce particular experiences;
lesions can remove specific capacities;
drugs alter perception and personality;
sleep, seizures, trauma, etc. systematically alter consciousness.
Kastrup doesn't deny these correlations. His interpretation is different.
He essentially says that the brain is what consciousness looks like from an external perspective, rather than the thing producing consciousness.
A useful analogy is a radio:
Damage the radio → the music disappears.
That doesn't necessarily mean the radio produced the music.
That's a legitimate philosophical possibility.
But critics respond:
Why should we believe the brain is a radio rather than a generator?
The correlation itself doesn't decide between the two. And the “radio” hypothesis doesn't currently generate an obvious experiment that distinguishes it from conventional neuroscience.
Kastrup explicitly addresses this objection in his work. (Reference Global)
Suppose consciousness is fundamental.
Then what exactly was happening before organisms evolved?
Kastrup's answer is essentially that consciousness was already there; organisms don't create consciousness but establish particular dissociated perspectives within it.
That is possible.
But evolution then becomes somewhat strange conceptually.
Why did natural selection produce increasingly sophisticated brains if brains aren't generating consciousness?
Kastrup can answer:
Evolution is selecting increasingly sophisticated representations/interfaces through which the underlying consciousness can express itself.
But critics will ask whether this is an independently motivated explanation or merely a reinterpretation of evolutionary biology.
The existence of a universe billions of years before humans is one of the classic objections Kastrup explicitly discusses. (Reference Global)
There's a potentially dangerous feature of Kastrup's theory:
Almost any observation can potentially be incorporated into it.
Brain activity changes → that's how dissociated consciousness manifests physically.
No brain activity → consciousness could still exist independently.
The universe existed before humans → the universal consciousness existed before humans.
Quantum mechanics looks strange → that's compatible with consciousness being fundamental.
Near-death experiences → potentially compatible with consciousness not depending on the brain.
This flexibility makes the theory philosophically powerful—but scientifically it can become a weakness.
A scientific theory gains credibility partly by making risky predictions that could turn out to be wrong.
So the question is:
What observation would make Kastrup say, “Okay, analytic idealism is probably false”?
If there isn't a good answer, then scientists have a legitimate reason to classify it as metaphysics rather than an empirical scientific theory.
Interestingly, Kastrup has explicitly discussed the question of whether analytic idealism is falsifiable. (YouTube)
Here's a subtle philosophical objection.
The evidence establishes something like:
We have conscious experiences, and we can describe stable regularities mathematically in terms of physical objects and processes.
Kastrup takes the additional step:
Therefore the physical world isn't fundamental; it is the appearance of mental processes.
But why?
There are intermediate possibilities.
For example, Russellian monism says that physics describes the mathematical/structural relationships between things without telling us what the underlying stuff actually is. Perhaps the intrinsic nature of the physical is experiential.
That's importantly different from Kastrup's idealism.
So one can accept the strongest criticism of materialism—namely, that physics doesn't explain why there is experience—without accepting universal consciousness as the ultimate ontology.
This is one reason contemporary philosophers have alternatives such as panpsychism, cosmopsychism and Russellian monism. (Stanford Encyclopedia of Philosophy)
This is a philosophical criticism rather than a scientific one.
Kastrup is quite persuasive when he attacks the idea that our subjective experience can simply be reduced to descriptions of neural activity.
But contemporary physicalism has sophisticated responses.
A physicalist doesn't necessarily have to say:
“Consciousness is an illusion.”
They can instead say that consciousness is a physical phenomenon whose nature isn't captured by our current conceptual vocabulary, or that phenomenal facts are necessarily connected to physical facts even though the connection isn't obvious from introspection.
The philosophy of physicalism is considerably more sophisticated than “brains are made of atoms, therefore consciousness magically appears.” (Stanford Encyclopedia of Philosophy)
So defeating naïve materialism doesn't automatically establish idealism.
This one is worth knowing because Goff is himself not a conventional hard-line materialist. He's sympathetic to panpsychism/Russellian approaches.
Goff has directly criticized Kastrup over the question of whether consciousness would be causally inert (epiphenomenal) under physicalism. Kastrup argues that physicalism has difficulty giving consciousness causal power; Goff disputes that argument. (Conscience and Consciousness)
That's significant because it means the debate isn't simply:
Kastrup vs. ignorant materialists.
It's more like:
Kastrup vs. several competing attempts to solve the same fundamental problem.
I'd put them roughly like this:
Very serious
Why does one universal consciousness divide into private minds?
Does idealism actually explain anything, or merely make consciousness fundamental?
What empirical observation could distinguish idealism from physicalism?
Serious but not necessarily fatal
4. The extraordinarily tight brain/consciousness correlations.
5. Explaining evolution and the apparent pre-human universe.
6. Whether the physical world really needs to be interpreted as
mental.
Less decisive
7. “Idealism sounds absurd/intuitive nonsense.”
That last one isn't much of an argument. Reality already violates plenty of our intuitions, and even the Stanford Encyclopedia notes that counterintuitiveness by itself isn't a good reason to reject views about consciousness. (Stanford Encyclopedia of Philosophy)
I actually think Kastrup's strongest argument isn't that idealism has been scientifically proven.
It's something more modest:
Physicalism has not explained why there is subjective experience in the first place, so we shouldn't treat physicalism as an established description of ultimate reality.
That's a much harder claim to knock down.
But there's a huge logical gap between:
“Physicalism has a deep explanatory problem.”
and
“Therefore Kastrup's universal-consciousness idealism is true.”
The first is a serious philosophical position. The second requires considerably more argument.
If you want to go one level deeper, the “one
consciousness / many minds” problem is probably the
place to focus. That's where Kastrup's theory gets genuinely
fascinating—and where I think the most difficult challenge to it
lies.
Yes. In fact, if you want to understand the strongest case for idealism, I wouldn't start with Kastrup alone. There are several philosophers whose arguments are, in my view, more rigorous in particular respects.
The important caveat is that they don't all defend Kastrup's specific analytic idealism. Some defend forms of idealism, cosmopsychism, or closely related positions.
John Foster is particularly interesting because he develops an explicitly analytic defense of idealism rather than relying on mystical or quantum-mechanical arguments.
His book The Case for Idealism (1982) argues that the most defensible account of perception ultimately makes the physical world dependent on experience. The contemporary Stanford Encyclopedia of Philosophy identifies Foster as one of the relatively few modern Anglophone philosophers to construct an explicit defense of idealism. (Stanford Encyclopedia of Philosophy)
His basic strategy is roughly:
1. What we directly encounter are experiences.
2. We infer an external physical world from those
experiences.
3. But the supposed physical world is never
encountered independently of experience.
4. Therefore, positing a fundamentally non-mental
physical substrate may be an unnecessary metaphysical addition.
That's considerably more sophisticated than “everything is consciousness.”
If your question is “Can idealism be defended using mainstream analytic-philosophical methods?” Foster is one of the best places to look.
Timothy Sprigge defended a form of absolute idealism/panpsychism.
His The Vindication of Absolute Idealism (1983) is another major modern defense. The SEP specifically groups Sprigge with Foster as one of the important modern Anglophone defenders of explicit idealism. (Stanford Encyclopedia of Philosophy)
Sprigge is especially interesting because he tries to construct an ontology in which experience is fundamental without reducing reality to individual human minds.
That gets directly into the problem we were discussing with Kastrup:
How can there be a single experiential reality while there are apparently many individual subjects?
Sprigge's approach is different from Kastrup's and is worth comparing.
Howard Robinson has defended idealism through arguments about perception and the nature of physical objects.
His Perception and Idealism (2022) is particularly relevant because it is a contemporary analytic treatment rather than an appeal to quantum physics or paranormal phenomena. The SEP lists it among the recent explicit defenses of idealism. (Stanford Encyclopedia of Philosophy)
If you want the argument:
“What exactly does perception give us reason to believe exists?”
rather than:
“What does quantum mechanics imply about consciousness?”
Robinson is probably more useful than Kastrup.
This is a slightly surprising recommendation.
Michael Huemer doesn't simply argue “idealism is true.” Instead, his work on phenomenal conservatism gives idealists a powerful epistemological weapon.
His principle is roughly:
If something seems to be the case, that gives you at least some prima facie justification for believing it, unless you have a reason to doubt it.
He has defended this principle extensively in analytic epistemology. (Wiley Online Library)
Why does this matter for idealism?
Because the idealist can say:
The one thing we know immediately is experience itself.
You don't infer that you're experiencing something. The experience is simply present.
By contrast:
“There exists a mind-independent physical world producing this experience”
is a theoretical inference.
That doesn't prove idealism. But it reverses the burden of proof in an interesting way.
Instead of asking:
“How can idealism possibly be true?”
you ask:
“Why should we believe in something fundamentally non-experiential when our evidence is fundamentally experiential?”
That's a much stronger formulation of the idealist challenge.
Donald Hoffman is probably the most interesting scientist adjacent to this discussion.
His “conscious realism” argues that consciousness is fundamental and that spacetime and physical objects aren't necessarily representations of ultimate reality. His argument draws heavily on mathematical models and evolutionary game theory.
This is superficially similar to Kastrup, but the intellectual route is quite different.
Hoffman's key move is essentially:
Evolution selects for fitness, not truth. Therefore, our perceptual interface may be systematically hiding the structure of objective reality.
That gives a scientifically motivated reason to question naïve physical realism.
But there's an important limitation: Hoffman's evolutionary arguments don't establish idealism by themselves. They establish that our perception of physical objects may not reveal ultimate ontology.
So:
Hoffman → challenges naïve realism.
Kastrup → argues for idealism as the alternative.
Those are different claims.
Galen Strawson is fascinating because he's attacking the problem from almost the opposite direction.
Strawson accepts something very important to the idealist:
Consciousness cannot plausibly emerge from genuinely non-conscious matter.
But instead of concluding:
“Therefore reality is fundamentally consciousness,”
he concludes approximately:
“Therefore matter itself must have an experiential/mental aspect.”
That's panpsychism.
And this creates a very useful three-way comparison:
| Position | Fundamental reality |
|---|---|
| Physicalism | Non-conscious physical stuff |
| Strawson-style panpsychism | Physical stuff with experiential nature |
| Kastrup-style idealism | Consciousness/experience |
The fascinating question is therefore not simply materialism vs. idealism.
It's:
Why should we choose idealism rather than panpsychism or Russellian monism once we reject naïve physicalism?
That's where the real philosophical battle is.
If you're genuinely interested in determining whether Kastrup is onto something, I'd go:
1. John Foster — The Case for Idealism
For the rigorous analytic argument.
2. Howard Robinson — Perception and Idealism
For a modern treatment of perception and idealism.
3. Bernardo Kastrup — The Idea of the World
For the most developed contemporary version of the specific
idealism we've been discussing. Kastrup's own academic
bibliography identifies this work alongside his technical papers
as the development of his position. (Bernardo
Kastrup)
4. Galen Strawson — Realistic Monism
To see the strongest alternative that accepts the basic
anti-materialist intuition without becoming an idealist.
5. Donald Hoffman — The Case Against Reality
For the empirical/evolutionary challenge to naïve physical
realism.
And I'd read Kastrup's “On the Plausibility of Idealism: Refuting Criticisms” alongside the objections I gave you earlier. It's specifically devoted to the brain-dependence objection, the pre-human universe objection, and other standard criticisms. (Reference Global)
I would not conclude from this list that idealism is secretly the consensus among philosophers. It isn't. Contemporary explicit idealism remains a minority position. The SEP describes Foster and Sprigge as notable precisely because explicit defenses of idealism have been relatively rare in modern Anglophone philosophy. (Stanford Encyclopedia of Philosophy)
But I would revise the impression one might get from my previous answer:
There is a much stronger philosophical case for idealism than “Kastrup says consciousness is fundamental.”
The deepest case starts from a very uncomfortable observation:
Everything you ever have evidence of is mediated by experience, while the supposedly fundamental non-experiential physical world is always something inferred from experience.
That doesn't establish idealism. But it's a genuinely difficult problem for the assumption that matter is obviously more fundamental than experience.
And that's where I think Foster + Robinson + Kastrup
make a considerably stronger case than Kastrup's more popular
presentations alone.
Yes—but there is an important distinction. There are physicists and philosophers who use quantum theory to motivate views that are congenial to idealism, but very few physicists would regard “quantum mechanics → idealism” as an established inference. The strongest work tends to be more modest: quantum mechanics undermines certain classical assumptions about an observer-independent physical world, and perhaps opens conceptual space for idealism.
I'd separate the candidates into three tiers.
If your criterion is “someone who actually knows quantum mechanics and thinks consciousness has fundamental ontological significance”, Henry Stapp is probably the first person I'd investigate.
Stapp was a theoretical physicist at Lawrence Berkeley National Laboratory and worked extensively on the foundations of quantum mechanics. His argument is not simply “the observer collapses the wave function.” He develops a fairly sophisticated interpretation inspired by von Neumann and Heisenberg in which conscious experience and quantum events have a fundamental role in the physical description. (Stanford Encyclopedia of Philosophy)
His paper Why Classical Mechanics Cannot Naturally Accommodate Consciousness But Quantum Mechanics Can is particularly relevant. He argues that certain mathematical characteristics of quantum theory make it substantially more hospitable to consciousness than classical mechanics. (arXiv)
And in Physicalism versus Quantum Mechanics, he makes the stronger philosophical argument that “physicalism” inherited from classical mechanics is not necessarily appropriate once the physical world is understood quantum mechanically. (arXiv)
That's a very interesting argument because it doesn't depend on the popular misconception that “consciousness causes wave-function collapse.”
However: Stapp does not straightforwardly establish Kastrup-style idealism. The SEP describes his inclusion of mental states as an essential part of quantum theory as highly speculative from the perspective of contemporary physics. (Stanford Encyclopedia of Philosophy)
So I'd rate him:
Physics: very serious
Quantum foundations: serious but heterodox
Idealism: sympathetic/compatible, but not a
conventional idealist
Acceptance among physicists: minority
His book Mindful Universe is probably the most accessible place to start. (Springer)
This is, paradoxically, the direction I'd recommend if your goal is to understand what quantum mechanics genuinely does to our conception of reality.
The central figures include Christopher Fuchs, Rüdiger Schack and David Mermin.
QBism says, very roughly, that a quantum state isn't an objective physical state sitting inside a system. It represents an agent's expectations about possible experiences resulting from interventions on the world.
That's remarkably close to some idealist intuitions:
Reality isn't simply a collection of objectively existing properties with definite values independent of interaction and perspective.
But QBism stops well short of:
Therefore reality is consciousness.
This distinction is crucial.
The Stanford Encyclopedia explicitly distinguishes QBism from Rovelli's relational quantum mechanics: experience and the agent are fundamental in QBism, whereas RQM treats the observer as just another physical system. (Stanford Encyclopedia of Philosophy)
And there is serious philosophical literature examining whether QBism can still count as a form of realism rather than mere subjectivism. (Springer)
So QBism is potentially much more interesting to a physicist than a direct “consciousness collapses the wave function” argument.
But:
QBism ≠ idealism.
It is better understood as a radical interpretation of what quantum states mean.
Carlo Rovelli's Relational Quantum Mechanics (RQM) is another important case.
Here the idea is that physical properties don't necessarily have absolute values independently of interactions. A property is defined relative to another physical system.
That sounds idealistic, but there's an enormous catch:
RQM explicitly does not require consciousness.
The “observer” can be an electron, detector, atom, etc. The Stanford Encyclopedia is unusually explicit about this: there is “nothing subjective, idealistic, or mentalistic” about RQM. (Stanford Encyclopedia of Philosophy)
Nevertheless, philosophically it's highly relevant to idealism because it undermines the classical picture of:
one objective world containing objects with fully determinate intrinsic properties, all of which exist absolutely from a God's-eye perspective.
RQM replaces that with something much more relational.
That gives an idealist an interesting argument:
Quantum physics has already forced physics away from the naïve classical picture. Perhaps the remaining step toward idealism isn't as radical as it initially appears.
But Rovelli himself isn't making that step.
If you're looking for someone who was actually a mainstream-caliber physicist/philosopher of physics and drew explicitly philosophical conclusions from quantum mechanics, Bernard d'Espagnat is worth reading.
He developed the notion of “veiled reality.”
The basic idea is that quantum mechanics may be telling us that the mathematical/observable structure we access experimentally isn't identical to the underlying reality itself.
That's not idealism—but it attacks the assumption that:
the world described by physics is literally the world as it exists independently of our modes of access to it.
That's a much more defensible starting point than saying “quantum mechanics proves consciousness creates reality.”
I think this is where the question becomes considerably more sophisticated.
Consider the following progression:
Classical realism
There is a world of physical objects possessing properties independently of observers.
↓
Quantum mechanics
The quantum state doesn't straightforwardly behave like a classical description of such objects.
↓
QBism
The quantum state represents an agent's expectations about experiences.
↓
Radical epistemology
Experience is epistemically primary; physical structure is something inferred from experience.
↓
Idealism
Perhaps experience isn't merely epistemically primary; perhaps it is ontologically primary.
The last arrow is not physics. It's a metaphysical inference.
But it's a much more intellectually defensible route to idealism than:
“The double-slit experiment proves consciousness creates reality.”
And importantly, contemporary scholarship does recognize serious philosophical connections between QBism, relational interpretations, perspectivalism and questions about scientific realism. (Springer)
Kastrup actually does try to make the quantum-to-idealism connection explicitly. He has a paper arguing that decades of quantum-physics evidence point toward the mental nature of the world, and he has described quantum mechanics as particularly naturally accommodated by idealism. (Bernardo Kastrup)
But this is precisely the part of Kastrup's work I'd be most cautious about.
His philosophical case for idealism is, in my opinion, considerably stronger than his claim that quantum physics provides evidence for idealism.
The quantum formalism itself doesn't require:
consciousness → wave-function collapse → idealism.
And some of the most philosophically radical interpretations of quantum mechanics—QBism and RQM—explicitly do not entail idealism. (Stanford Encyclopedia of Philosophy)
I'd rank them approximately:
| Approach | Quantum physics | Consciousness | Idealism |
|---|---|---|---|
| Henry Stapp | ⭐⭐⭐⭐⭐ | ⭐⭐⭐⭐⭐ | ⭐⭐⭐ |
| QBism / Fuchs | ⭐⭐⭐⭐⭐ | ⭐⭐⭐ | ⭐⭐ |
| Rovelli / RQM | ⭐⭐⭐⭐⭐ | ⭐ | ⭐ |
| d'Espagnat | ⭐⭐⭐⭐⭐ | ⭐ | ⭐⭐ |
| Kastrup | ⭐⭐⭐ | ⭐⭐⭐⭐⭐ | ⭐⭐⭐⭐⭐ |
The funny result is that Kastrup is the strongest idealist but not the strongest quantum physicist, whereas Stapp is almost the inverse.
If what you're really asking is:
“Is there a formulation of idealism that takes modern quantum theory seriously enough that a physicist couldn't simply dismiss it as pseudoscience?”
then yes. But I'd look beyond arguments about consciousness causing measurement.
The really interesting territory is quantum information, QBism, relational quantum mechanics, ontic structural realism, and the idea that spacetime/physical objects might be emergent rather than fundamental.
That leads to a considerably stronger—and much more subtle—case for something idealism-adjacent than the usual popular “quantum consciousness” literature.
If you'd like, I can lay out the strongest possible quantum-physics-based argument for idealism, step by step, and then show exactly where a physicist would object to each step. That would probably give you a much better sense of whether the quantum argument genuinely works.
Yes. If I were trying to construct the strongest
quantum-physics-based case for idealism, I would not
argue that “consciousness collapses the wavefunction.” That is
historically interesting, but it is too easy for a physicist to
reject.
I'd build the case in layers, and I'd distinguish very carefully between what quantum physics actually establishes and the metaphysical extrapolation.
The classical picture is roughly:
There is a world made of objects. Those objects possess properties. Observers discover those properties.
For example, an electron would be imagined as having a definite position and momentum whether or not anyone looks.
That picture is not simply carried over unchanged into quantum mechanics.
Quantum theory tells us that noncommuting quantities cannot generally be assigned simultaneous definite values, and different experimental arrangements yield different possible outcomes. Different interpretations disagree about what this means ontologically, but the classical picture of objects carrying a complete set of observer-independent properties is no longer straightforwardly available.
This is the first legitimate opening for idealism.
Physicist's objection:
"Fine, but abandoning classical realism does not imply idealism.
There are many alternatives."
Absolutely. This gets us only to:
Classical materialism is not the only possible ontology.
It doesn't get us to consciousness.
Consider the wavefunction.
In some interpretations, it is a physically real entity. In others, it represents information, expectations, dispositions, relations, etc.
For example, QBism treats quantum probabilities as an agent's personal degrees of belief about possible experiences resulting from interventions. The quantum state isn't simply a physical object sitting inside the world waiting to be discovered. (Stanford Encyclopedia of Philosophy)
Likewise, relational quantum mechanics says that physical properties are relative to interactions between systems, rather than being absolute properties possessed independently of everything else. (Stanford Encyclopedia of Philosophy)
This suggests a radically different picture:
Perhaps physics isn't describing a collection of intrinsically existing objects with properties. Perhaps it is describing the structure of possible experiences/interactions.
That's very congenial to idealism.
Physicist's objection:
"You're confusing an epistemic statement with an ontological one."
And this is a very strong objection.
From:
"The quantum state represents my expectations"
you cannot infer:
"Reality consists of experiences."
QBism itself does not make that inference. Indeed, QBism is deliberately careful about what it claims.
So this step is suggestive, not demonstrative.
The quantum formalism has two apparently different kinds of evolution.
The Schrödinger equation gives continuous, deterministic evolution:
[
|\psi(t)\rangle = U(t)|\psi(0)\rangle
]
But measurement gives us a definite outcome.
If an electron is in a superposition,
[
|\psi\rangle = a|0\rangle+b|1\rangle
]
ordinary unitary evolution can produce an entangled state such as
[
a|0\rangle|D_0\rangle+b|1\rangle|D_1\rangle.
]
Yet we experience one definite result.
That is the measurement problem.
And here we encounter something philosophically interesting:
The mathematical formalism gives us possibilities and correlations, while our world of experience contains definite events.
Some physicists—including Henry Stapp—have argued that this makes consciousness relevant to the ontology of quantum theory. Historically, von Neumann, London/Bauer and Wigner explored versions of the idea that conscious observation might complete the measurement process. (Stanford Encyclopedia of Philosophy)
The idealist interpretation would be:
Perhaps consciousness isn't an inexplicable byproduct of a world of definite physical facts. Perhaps definite physical facts are themselves aspects of a deeper experiential reality.
That's much more radical.
But again:
Physicist: "Decoherence + Everett solves this without consciousness."
Or:
Physicist: "Objective-collapse theories solve it physically."
Or:
Physicist: "There isn't actually a special physical collapse; you've misunderstood what the formalism is saying."
And these are perfectly serious responses.
So the measurement problem does not establish idealism.
This is where I think the argument becomes much stronger.
Take relational quantum mechanics.
Rovelli's interpretation says that an event is not simply:
"The electron has spin-up."
It is more like:
"The electron has spin-up relative to system A."
Different physical systems can therefore have different descriptions of the same quantum event, without this necessarily constituting a contradiction. (Stanford Encyclopedia of Philosophy)
This creates a fascinating philosophical possibility.
The universe may not fundamentally consist of:
objects → intrinsic properties → observers who discover them.
It might instead fundamentally consist of:
events → relations → perspectives.
That is profoundly different from classical substance metaphysics.
And an idealist can say:
"Exactly. What we call physical objects are stable structures within experience/relations, rather than fundamentally existing substances behind experience."
This is a respectable philosophical move.
But Rovelli himself would object:
"I don't need consciousness."
And he's right.
RQM explicitly treats any physical system as capable of playing the role of observer. An electron can be an "observer" in the relevant technical sense. Consciousness isn't required. (Stanford Encyclopedia of Philosophy)
So quantum relationality gets you toward anti-classical realism, but not necessarily toward idealism.
Here's where I think idealism has its strongest quantum-adjacent move.
Suppose we accept three things:
A. Physics describes relations, structures and probabilities extremely successfully.
B. Quantum theory doesn't straightforwardly tell us that the underlying reality consists of classical objects possessing intrinsic properties.
C. The one thing whose existence is directly undeniable is experience itself.
Now ask:
What is the ontology behind the mathematical structure of physics?
There are several possibilities.
The underlying reality is non-experiential physical stuff.
Physics describes the mathematical/structural properties of reality, but its intrinsic nature might be something different—and perhaps experiential.
Physical entities have some primitive experiential properties.
Experience isn't a property of the underlying reality.
Experience is the underlying reality.
And quantum mechanics arguably makes the fourth option less absurd than classical physics did.
That is, I think, the strongest quantum argument for idealism.
Notice how different this is from:
"The observer collapses the wavefunction."
It's much more subtle.
This is where I'd bring in Russellian ideas.
Modern physics gives us extraordinary knowledge of:
mathematical relationships;
symmetries;
dynamics;
probabilities;
interaction structure.
But what is the intrinsic nature of whatever instantiates those structures?
Physics is remarkably silent about that question.
This is sometimes called the structural/intrinsic distinction.
And now consciousness becomes interesting because:
We know one intrinsic reality directly: experience.
You don't infer that pain exists from equations. You experience it.
So the idealist can propose:
Maybe what physics describes mathematically from the outside is the relational structure of something whose intrinsic nature is experiential.
That's actually closer to Russellian monism than straightforward Kastrup-style idealism.
And this is, in my opinion, far more difficult for a physicist to dismiss than "quantum mechanics proves consciousness."
Here's the important part.
A really good physicist could respond to the entire argument approximately like this:
Quantum mechanics predicts experimental outcomes extraordinarily well.
It doesn't follow that its mathematical structures are literally the structure of consciousness.
That's correct.
This is probably the single strongest objection.
Decoherence occurs through physical interactions. Measurement-like behavior occurs when quantum systems interact with other quantum systems.
Nothing in the mathematics says:
[
\text{quantum interaction} \rightarrow \text{conscious observer}.
]
RQM explicitly removes consciousness from the measurement story. (Stanford Encyclopedia of Philosophy)
And there are interpretations—Everettian, Bohmian, objective-collapse, etc.—that don't need consciousness at all.
This is devastating against a quantum argument.
You can show that quantum mechanics doesn't require classical metaphysics.
You can show that the wavefunction is ontologically controversial.
You can show that relationality is fundamental in some interpretations.
But none of that demonstrates:
[
\text{fundamental reality}=\text{consciousness}.
]
That's an additional metaphysical hypothesis.
This is probably the most interesting alternative.
Perhaps the fundamental stuff is information or relations.
Then consciousness emerges from sufficiently complex information-processing structures.
Or perhaps consciousness and physical reality are two aspects of something more fundamental.
This is why people like neutral monists and Russellian monists occupy an important middle ground.
The idealist has to explain why experience gets ontological priority over information, structure, or some unknown neutral substrate.
This is the big empirical challenge.
We can alter:
visual cortex → visual experience changes;
temporal lobe → memory/personality changes;
anesthesia → consciousness disappears;
psychedelics → experience changes dramatically;
stimulation → specific experiences can occur.
And these relationships are systematic.
The physicist says:
"Why invoke universal consciousness when neuroscience already gives us an increasingly detailed account of the mechanisms?"
The idealist responds:
"Because mechanisms explain correlations and transformations of experience, not why there is experience in the first place."
That's a legitimate response.
But it means the debate has moved back from physics into philosophy of mind.
If I had to formulate the strongest intellectually defensible argument, I'd make it something like this:
Classical physical realism—the idea that reality consists of observer-independent objects with definite intrinsic properties—is not straightforwardly compatible with quantum mechanics.
Several serious interpretations of quantum mechanics replace that picture with something more relational, perspectival, dispositional or informational.
Physics gives us extraordinarily precise descriptions of the mathematical/relational structure of physical phenomena, but arguably does not tell us what the underlying reality is intrinsically.
Conscious experience is the one intrinsic reality we encounter directly rather than infer theoretically.
Therefore, it is philosophically reasonable to investigate the possibility that the intrinsic nature underlying physical structure is experiential.
Idealism—or at least Russellian monism—is a serious candidate for the metaphysical interpretation of quantum physics.
But notice what I deliberately didn't say:
"Quantum physics proves idealism."
It doesn't.
I'd separate his argument into two independent arguments:
Argument A — philosophical
Consciousness is fundamental because physicalism cannot explain experience.
Argument B — quantum
Quantum mechanics is naturally compatible with a non-classical, relational, non-object-based ontology.
Then use B to remove an obstacle to A, rather than claiming B proves A.
That's much stronger.
In schematic form:
[
\boxed{\text{Quantum mechanics undermines naive physical realism}}
]
doesn't imply
[
\boxed{\text{therefore idealism}}
]
but it potentially supports:
[
\boxed{\text{physicalism is not the metaphysical default it
appeared to be}}
]
And then the hard problem of consciousness becomes an independent reason to investigate idealism.
If I were debating a physicist, I would not defend Kastrup by saying "quantum mechanics proves consciousness creates reality." I'd probably lose immediately.
I'd defend a much weaker—and considerably stronger—claim:
Quantum theory has made the old picture of a self-subsistent classical material world less compelling. It leaves open a metaphysics in which the mathematical structures described by physics are not themselves the ultimate furniture of reality. Since conscious experience is the only intrinsic aspect of reality directly given to us, idealism or Russellian monism becomes a legitimate candidate for what lies behind the formalism.
A physicist can still reject that argument. But to do so, they have to engage with metaphysics and the philosophy of mind, rather than simply saying "quantum mechanics doesn't involve consciousness."
And that's a much more interesting debate.
One final caveat: contemporary philosophical surveys are quite explicit that quantum-consciousness proposals vary enormously in rigor; vague appeals to entanglement, superposition, etc. without a precise physical mechanism aren't considered scientific progress. Stapp's approach is taken more seriously because it engages with actual quantum formalism, but even the SEP characterizes the addition of mental states as highly speculative by current physics standards. (Stanford Encyclopedia of Philosophy)
The difference is subtle but extremely important. Russellian monism and idealism agree on a major criticism of physicalism, but disagree about what lies underneath the physical structure described by physics.
A useful one-line distinction is:
Russellian monism: the intrinsic nature of matter may be experiential.
Idealism: there isn't fundamentally a non-mental matter underneath experience; experience is fundamental reality.
Both positions take seriously a problem with ordinary physicalism.
Physics tells us extraordinarily precisely about structure and relations: mass, charge, spin, fields, dynamics, etc.
But physics arguably doesn't tell us what these things are intrinsically.
Bertrand Russell famously pointed out a version of this problem: physics gives us the mathematical/structural description of matter, while the intrinsic nature of whatever has those properties seems largely opaque.
That creates an opening.
Imagine reality is like this:
Underlying reality
→ has an intrinsic nature we don't directly perceive
→ has relational/structural properties described by physics
→ those intrinsic properties are, perhaps, experiential
So you might have:
One underlying stuff → two descriptions
From the outside:
physics: mass, charge, spin, fields, causal relations...
From the inside:
experience: phenomenal qualities.
This is why Russellian monism is sometimes described as a dual-aspect or intrinsic-nature approach.
Importantly, the fundamental stuff is not necessarily consciousness as we experience it. It might have primitive experiential properties that combine or organize into consciousness.
Galen Strawson is one prominent contemporary defender of a version of this general family of views.
Idealism makes a stronger claim.
It says:
There isn't a fundamentally non-experiential substance underneath experience.
Reality is fundamentally mental/experiential.
The physical world is then something like:
experience → stable patterns → represented as physical objects
So the direction of explanation is reversed.
For a Kastrup-style idealist:
Brain activity doesn't ultimately produce consciousness.
Instead:
What we call "brain activity" is the external appearance of processes occurring within consciousness.
That's a much more radical ontological claim.
Matter → consciousness
The physical world is fundamental.
Unknown intrinsic reality
↙︎ ↘︎
physical structure experience
The physical and experiential are two aspects of something deeper.
Consciousness / experience
↓
physical world as its structured appearance
The experiential is fundamental.
It has a clever response to the hard problem.
Suppose physics tells us everything about the brain:
neurons
ion channels
neurotransmitters
electrical activity
computational processes
You can still ask:
Why does any of that feel like something?
The Russellian says:
Perhaps you've been looking for the wrong thing. Physics describes the structure of the brain, but it doesn't describe its intrinsic nature. The intrinsic nature of the relevant physical processes may already be experiential.
So consciousness isn't something that mysteriously emerges from completely non-conscious ingredients.
It was there at the fundamental level.
This is one reason Galen Strawson argues that genuine physicalism may actually require something like panpsychism.
It's basically the combination problem.
Suppose electrons have incredibly primitive experiential properties.
How do billions of neurons—and ultimately trillions of microscopic physical processes—combine into one unified human consciousness?
Why do your experiences form one subject rather than an enormous collection of microscopic subjects?
That's extremely difficult.
Idealism has an interesting advantage here.
Kastrup can say:
There aren't billions of fundamentally separate little consciousnesses that need to combine. There is fundamentally one consciousness, and individual minds are dissociated regions of it.
So idealism attacks the combination problem by reversing the direction:
Panpsychism:
many tiny experiences → one human experience
Kastrup:
one universal consciousness → many individual experiences
But now Kastrup gets the decomposition/dissociation problem I mentioned earlier:
How does one consciousness produce genuinely separate private subjects?
Suppose you completely understood the physical structure of a brain.
"You still wouldn't know the intrinsic nature of that structure. Perhaps it's experiential."
"The brain itself is already an appearance within consciousness. There isn't another underlying physical thing whose intrinsic nature we need to discover."
That's a much stronger claim.
This is why I think Russellian monism is actually a particularly interesting bridge between physics and idealism.
Quantum mechanics may suggest that what physics gives us is fundamentally a description of:
relations,
dispositions,
probabilities,
mathematical structure,
information.
But it may not tell us what the underlying reality is in itself.
Russellian monism says:
Fine. Perhaps the intrinsic nature of that structure is experiential.
Idealism then makes one additional step:
Why posit an underlying non-experiential substrate at all? Perhaps the experiential reality itself is fundamental.
So you could imagine a spectrum:
Physicalism
Fundamental reality = non-experiential physical stuff.
↓
Russellian monism
Fundamental reality = something whose structure is physical but whose intrinsic nature may be experiential.
↓
Cosmopsychism
The universe as a whole has a fundamental experiential nature.
↓
Idealism
Fundamental reality is experiential; the physical world is a representation/appearance of it.
The boundaries between these positions can get blurry, and different philosophers use the terminology differently.
Russellian monism.
It makes a smaller ontological leap.
It can say:
"Physics is completely correct about its domain. I'm simply arguing that its equations don't exhaust the intrinsic nature of what they describe."
A physicist can be much more comfortable with that.
Idealism asks for a larger reversal:
"What physics calls matter isn't fundamental at all; it is an appearance within consciousness."
That's considerably harder to connect to established physics.
That's debatable.
Russellian monism has a very attractive answer:
Experience doesn't emerge from non-experience because the intrinsic nature of the physical was experiential all along.
But it faces the combination problem.
Idealism has an equally attractive answer:
There is no emergence problem because experience is fundamental.
But it faces the problem of explaining why one universal consciousness appears as many private minds and why the physical world behaves with such extraordinary regularity.
So there's a fascinating symmetry:
| Russellian monism | Idealism | |
|---|---|---|
| Fundamental | Unknown/neutral reality | Experience |
| Physical world | Real structure of fundamental reality | Appearance/representation |
| Consciousness | Intrinsic aspect of reality | Fundamental reality |
| Hard problem | Avoided by intrinsic nature | Dissolved by making consciousness primitive |
| Main problem | Combination | Dissociation / individual minds |
| Compatibility with physics | Very high | More speculative |
Once you've accepted that physics may describe structure without revealing intrinsic nature, there are really two very different ways to proceed:
Russell:
"There is something underneath the structure. We don't know
what it is; perhaps it is experiential."
Idealist:
"Why are we inventing something underneath experience at all?
Experience is what we know directly; perhaps the physical
structure is what that experience looks like from a particular
perspective."
That second move is essentially where Kastrup departs from Russellian monism.
And that is where the really difficult philosophical argument begins—not with quantum mechanics itself, but with the question:
Does the fact that experience is directly given justify treating experience as ontologically fundamental?
That's, in my view, the strongest question to put to both physicalism and idealism.
You're right. That is the most important objection to the line of argument I gave you, and I should have addressed it explicitly. Carroll's challenge goes deeper than “quantum mechanics doesn't prove idealism.”
I think his objection can be sharpened into a serious dilemma for Goff, and also for some versions of Russellian monism.
Carroll's own 2021 paper makes essentially this point: physics gives us very strong evidence for causal closure in the regime relevant to brains, so if intrinsically mental properties are added to fundamental ontology, either they modify the known dynamics or it becomes difficult to see how they do explanatory work. (PhilArchive)
Suppose we say:
Consciousness is fundamental.
Carroll asks:
Does this make any difference to what happens physically?
There are two possibilities.
Then we need something like:
[
\text{physical state} + \text{consciousness}
\longrightarrow
\text{different physical evolution}
]
In other words, consciousness has causal influence.
But then there must be some additional law specifying that influence.
For example, perhaps:
[
\text{brain state } B
\quad\Rightarrow\quad
\text{conscious state } C
]
and
[
C \quad\Rightarrow\quad \text{some alteration in physical
evolution}.
]
That's a genuine scientific hypothesis.
But now we have a problem:
Where is the evidence for the additional law?
Modern physics already describes the dynamics of matter making up brains extremely successfully. Carroll argues that there is little room for an additional causal influence without substantially modifying known physics. (PhilPapers)
And if consciousness changes physical outcomes, we should in principle be able to find some experimental signature.
Then we have:
[
\text{physical state } P(t)
\rightarrow
P(t+1)
]
exactly as ordinary physics says.
Consciousness is somehow "there," but it doesn't affect anything.
Then Carroll asks:
What explanatory work is consciousness doing?
Suppose the brain is in exactly the same physical state whether consciousness exists or not.
The neurons fire identically.
The muscles move identically.
The person says:
"I am conscious."
Everything is physically identical.
Then the consciousness seems to be causally irrelevant.
This is basically the problem of epiphenomenalism.
And Carroll's criticism is devastating if the panpsychist's claim is supposed to explain why consciousness makes a difference.
I previously said:
Physics describes structure; perhaps consciousness is the intrinsic nature underlying that structure.
That's a legitimate Russellian-monist proposal.
But Carroll can immediately ask:
Fine. What does the intrinsic nature do?
If the answer is:
Nothing beyond what the physical structure already does,
then the experiential aspect looks like an ontological passenger.
If the answer is:
It causes physical events,
then we need psychophysical laws.
That is the dilemma I didn't adequately confront.
I think Goff has a much better answer than simply saying “consciousness causes things.”
He can say:
You're assuming that causal explanation has to operate by adding another force to the equations of physics.
Maybe that's the wrong conception.
Consider a simple analogy.
The laws of molecular physics explain the behavior of gas molecules.
But pressure is real.
Temperature is real.
Entropy is real.
You don't necessarily add a new fundamental force corresponding to “pressure” to the Standard Model.
Instead:
[
\text{microphysical dynamics}
\rightarrow
\text{higher-level regularities}
]
So perhaps consciousness is a higher-level, real feature of certain physical systems without requiring a new fundamental force.
Carroll himself makes essentially this kind of move: he argues that consciousness can be real without being fundamental, just as macroscopic properties such as those of tables or thermodynamic quantities are real without requiring modifications to fundamental physics. (Lex Fridman)
But notice the problem for Goff:
If consciousness is fundamental, this analogy becomes less straightforward.
If it is merely an emergent higher-level property, Carroll wins.
Goff's position isn't necessarily:
Consciousness is a new force that pushes particles around.
Instead, he wants to question what physics is telling us about the intrinsic nature of the physical world in the first place.
This is the point he makes in discussions with Carroll: physics gives us mathematical/causal descriptions of what matter does, but perhaps doesn't tell us what matter is intrinsically. Goff therefore argues that panpsychism can be an interpretation of physics rather than a competing physical theory. (IAI TV)
That creates an interesting possibility:
Consciousness doesn't intervene in physical dynamics because consciousness isn't something sitting alongside the physical dynamics and pushing them around.
Rather:
The physical dynamics are the mathematical/structural description of processes whose intrinsic nature is experiential.
If that's right, Carroll's demand for a new force might be misplaced.
He can say:
"Fine. But then you've changed the meaning of 'fundamental consciousness.'"
Suppose the complete physical state of the universe is (P).
And the consciousness state is (C).
If:
[
P(t) \rightarrow P(t+1)
]
is completely determined by physics, and (C) never enters that equation, then C doesn't explain anything about the evolution of P.
It's simply another description of the same underlying process.
And then Carroll can ask:
Why call consciousness fundamental rather than epiphenomenal?
This is where the debate gets really interesting.
I think this is the distinction that was missing from my previous answer.
Something can be ontologically fundamental without being a new causal force.
For example, imagine reality consists fundamentally of:
[
X
]
and X has two inseparable aspects:
[
X =
\begin{cases}
\text{structural aspect}\
\text{experiential aspect}
\end{cases}
]
The physical equations describe the structural aspect.
The experiential aspect doesn't constitute an additional force.
So the idealist/Russellian can say:
"I'm not proposing that consciousness changes the equations. I'm saying that the equations describe only one aspect of what exists."
That's coherent.
But then we need to ask:
What does “aspect” actually mean?
If it makes no possible difference whatsoever, Carroll can reasonably say:
"Then you're making an empirically empty metaphysical distinction."
And that is a serious objection.
This is one reason your original question about Kastrup becomes particularly interesting.
Goff's panpsychism typically says something like:
Consciousness is a fundamental aspect of physical reality.
Kastrup's idealism says something much stronger:
Physical reality itself is derivative of consciousness.
That's a crucial difference.
For Kastrup, consciousness doesn't need to causally modify physics because physics isn't ontologically prior to consciousness.
The relationship is more like:
[
\boxed{\text{consciousness}}
]
↓
[
\boxed{\text{physical world}}
]
rather than:
[
\boxed{\text{physical world}}
+
\boxed{\text{consciousness}}
]
with consciousness occasionally pushing the physical world around.
This potentially avoids Carroll's dilemma.
Carroll can simply turn the question around:
What determines the physical laws?
If everything is ultimately consciousness, why does consciousness generate precisely these extraordinarily precise mathematical regularities?
Why:
[
\text{Schrödinger equation}
]
rather than some other dynamics?
Why quantum field theory?
Why the Standard Model?
Why does the universe exhibit such extraordinary mathematical consistency?
Kastrup has to explain why the universal consciousness manifests as an extraordinarily lawlike physical world.
And that's not trivial.
So we get:
How does fundamental consciousness matter causally without changing physics?
Why does fundamental consciousness generate this particular extraordinarily stable physical structure?
Those are different problems.
This is where I would now modify my earlier argument substantially.
Quantum mechanics doesn't solve Carroll's challenge.
Rather, it gives us a reason to question whether the physical description is the whole ontology.
For example, if one adopts a relational or QBist interpretation, the quantum state may not represent a simple observer-independent physical object with intrinsic properties. That opens conceptual room for a metaphysics in which physics describes structure rather than ultimate substance.
But:
[
\text{quantum relationality}
\not\Rightarrow
\text{consciousness}.
]
And more importantly:
[
\text{quantum relationality}
\not\Rightarrow
\text{causal efficacy of consciousness}.
]
So Carroll's objection survives the quantum argument.
And this is where I would now place the debate.
[
\text{physics}
\rightarrow
\text{complete causal dynamics}
]
Consciousness is a real emergent phenomenon.
No modification to fundamental physics is required.
The remaining challenge is explaining why the physical processes feel like anything.
[
\text{physics}
\rightarrow
\text{structure/dynamics}
]
plus
[
\text{fundamental experiential properties}.
]
But then Goff has to explain either:
why those properties don't need to alter physical dynamics, or
what psychophysical principles connect them to physical systems.
This is exactly where Carroll presses him.
[
\text{consciousness}
\rightarrow
\text{physical world as appearance/representation}.
]
Here consciousness doesn't need to be an extra causal variable in physics.
Instead, physics describes the regularities of the appearance.
This is conceptually much more elegant with respect to Carroll's objection.
But it has its own enormous burden:
Why does universal consciousness generate a world with precisely the mathematical structure described by physics, and why are individual minds so tightly correlated with particular brains?
No.
He exposes a serious explanatory problem.
Goff can consistently say:
"I don't claim consciousness is an additional force."
But then Carroll can ask:
"Then what explanatory work does its fundamental status perform?"
Goff's answer is essentially:
"It explains the intrinsic nature of reality rather than its causal dynamics."
That's philosophically coherent.
But Carroll can respond:
"Then you've given an answer to a metaphysical question while leaving the causal/explanatory question untouched."
And I think that's a genuinely strong criticism.
I was perhaps too favorable to it in my previous answer.
Russellian monism has a very attractive answer to:
Why is there experience?
But it has a potentially ugly answer to:
Why does experience do anything?
If the experiential aspect is causally inert, it looks epiphenomenal.
If it isn't causally inert, we need psychophysical laws.
And if the answer is that the physical and experiential are simply two descriptions of the same underlying reality, then we have to explain why the physical description is causally complete while the experiential description is nevertheless ontologically fundamental.
That's possible, but it's not free.
Carroll's challenge forces us to distinguish two very different questions:
Question 1:
Why does matter behave according to these equations?
Question 2:
Why is there something it is like to be a system whose behavior is described by these equations?
Physicalism says:
Eventually, Question 2 will be answered in terms of Question 1.
Goff says:
It cannot be.
Kastrup says:
The direction of explanation should be reversed.
And Russellian monism says:
Neither should be reduced to the other; both are aspects of something deeper.
Quantum mechanics doesn't decide between these three.
And that's the point I should have made more clearly before.
If anything, Carroll's challenge tells us that the best argument for idealism cannot simply be “physics leaves room for consciousness.” It has to explain why consciousness is ontologically prior without either becoming a new force or becoming causally irrelevant.
That is a much harder—and much more interesting—problem.
I do see the attraction. In fact, if one wants a version
of idealism in which consciousness is both fundamental and
causally efficacious, consciousness-induced collapse is one of
the cleanest hypotheses available.
What I was resisting earlier was not the logical coherence of the idea, but the claim that it is currently the best-supported physical theory. Those are different questions.
Suppose the quantum state evolves as
[
|\psi\rangle \rightarrow a|A\rangle+b|B\rangle.
]
Then, on a consciousness-collapse theory, a conscious experience selects one outcome:
[
a|A\rangle+b|B\rangle
\quad\longrightarrow\quad
|A\rangle
]
or
[
|B\rangle.
]
Now consciousness isn't epiphenomenal. It has a genuine causal role in determining which physical history actually occurs.
That gives you exactly what Carroll is demanding:
If consciousness is fundamental, it should make a difference to physical evolution.
And unlike a vague “psychophysical interaction,” quantum mechanics already contains a notorious place where an extra ingredient is needed: the measurement problem.
So philosophically, the fit is extremely elegant.
It potentially gives you a very attractive hierarchy:
1. Fundamental consciousness
↓
2. Quantum possibilities
↓
3. Conscious experience / selection of an outcome
↓
4. Classical physical reality
That is remarkably congenial to idealism.
Instead of consciousness being something that appears inside a pre-existing classical universe, the classical universe itself emerges from a deeper interaction between quantum possibilities and consciousness.
That's a much more radical and interesting proposal than simply adding “consciousness” to the inventory of physical properties.
And historically, this isn't a crazy idea invented by New Age writers. von Neumann, London and Bauer, Wigner, and later Henry Stapp all explored versions of a connection between consciousness and quantum measurement. Stapp in particular has developed a sophisticated version within quantum foundations. The Stanford Encyclopedia's treatment of consciousness and quantum theory discusses these proposals explicitly.
Because there are three enormous problems.
This is the fundamental problem.
The standard quantum formalism has a measurement problem, but there are multiple ways of addressing it:
Everett: no collapse; all branches persist.
Bohm: particles have definite configurations guided by the wavefunction.
GRW/CSL: collapse happens spontaneously according to modified physical dynamics.
Relational approaches: the apparent collapse is relative to physical interactions.
QBism: collapse represents an agent's updating of expectations.
So:
Measurement problem → therefore consciousness collapses the wavefunction
is not a deduction.
It's:
Measurement problem → perhaps consciousness is the missing ingredient.
That's a legitimate hypothesis, but there are competitors.
This is where Carroll's objection comes back.
Suppose consciousness causes collapse.
Then we need to specify when and how.
Does any conscious experience collapse a wavefunction?
Does a dog?
An insect?
A single neuron?
A sufficiently complex AI?
What about an unconscious person under anesthesia?
What precisely counts as the relevant consciousness?
And mathematically:
[
\text{What term gets added to the quantum dynamics?}
]
Without an answer, “consciousness collapses the wavefunction” isn't yet a physical theory.
It's a research program.
And this is precisely where Henry Stapp's work becomes interesting: he attempts to formulate an actual mechanism involving quantum measurement and conscious choice rather than merely asserting that consciousness causes collapse. But even sympathetic philosophical treatments characterize such proposals as speculative relative to current physics.
Suppose:
[
|\psi\rangle =
\frac{1}{\sqrt2}|A\rangle+
\frac{1}{\sqrt2}|B\rangle.
]
Consciousness collapses it.
Fine.
Why A rather than B?
If consciousness is making the selection, then we have to explain why the observed frequencies obey the Born rule:
[
P(A)=|a|^2,\qquad P(B)=|b|^2.
]
If consciousness is genuinely intervening, you'd naturally expect the possibility of deviations from the Born rule.
But quantum experiments have tested quantum probabilities with extraordinary precision.
So the idealist needs something like:
Consciousness causes collapse, but consciousness is constrained to produce exactly Born-rule statistics.
That's possible.
But now you need another psychophysical law.
And the deeper the theory gets, the more it begins to look like:
[
\text{quantum mechanics}
+
\text{a very carefully tuned consciousness law}.
]
A physicist will reasonably ask:
Why introduce this extra machinery when GRW or another objective-collapse theory can modify the physics without invoking consciousness?
And this is where I think the idealist has something genuinely interesting to say.
The objective-collapse theories have to introduce new physical parameters anyway.
GRW, for example, introduces spontaneous localization with characteristic scales/rates.
So the choice isn't necessarily:
“Standard physics” vs. “weird consciousness.”
It can be:
Which additional ontology/law best explains why definite outcomes occur?
One candidate:
Objective physical collapse.
Another:
Consciousness-mediated collapse.
The latter has an advantage if you already think consciousness is fundamental: it unifies two otherwise mysterious facts:
why there are definite outcomes;
why there is conscious experience.
That is a real theoretical virtue.
I previously suggested that Kastrup's idealism might avoid Carroll's challenge because consciousness doesn't need to causally intervene in physics.
I think that's too easy an escape.
If you're going to claim that consciousness is fundamental and that it explains physical reality, then a theory in which consciousness actually participates in quantum state reduction has an important advantage:
It makes the ontological claim physically consequential.
That's much more satisfying than saying:
“Consciousness is fundamental, but the entire physical universe would evolve exactly the same way if consciousness didn't exist.”
The latter position is vulnerable to Carroll's criticism of explanatory redundancy.
Because Kastrup's idealism doesn't need it, and in some respects his ontology is actually incompatible with the usual consciousness-causes-collapse picture.
In Kastrup's view, physical systems are appearances of mental processes.
So the relationship isn't fundamentally:
[
\text{consciousness}
\rightarrow
\text{physical matter}.
]
It's closer to:
[
\text{mental reality}
\rightarrow
\text{physical appearance}.
]
A collapse event would therefore not necessarily be something a conscious observer does to an external physical world. The physical event is already an appearance within consciousness.
This is why Kastrup generally doesn't build his idealism around Wigner-style observer-induced collapse.
And philosophically, I think he's right to be cautious.
Suppose the true ontology is:
[
\boxed{\text{consciousness}}
]
and quantum mechanics describes the probabilistic structure of conscious experience.
Then what we call:
“wavefunction collapse”
could actually be the transition from a set of possible experiences to one actual experience.
In other words, collapse isn't:
consciousness causing physical reality.
It is:
the mathematical representation of consciousness becoming definite from a particular perspective.
That's closer to QBism and some relational approaches.
But now we've lost the causal role you're looking for.
Consciousness is fundamental, but doesn't modify physical dynamics.
Problem: Carroll's causal/explanatory redundancy objection.
Consciousness is fundamental and causes quantum collapse.
Advantage: consciousness has genuine physical consequences.
Problem: needs a precise psychophysical law and an explanation of the Born rule.
Physical reality is itself a structured appearance of fundamental consciousness; collapse is not something consciousness does to matter.
Advantage: no dualistic interaction problem.
Problem: harder to connect to actual physics and risks becoming empirically indistinguishable from ordinary interpretations.
Of these three, I agree with you that B is the most attractive if the objective is specifically to make consciousness both fundamental and causally efficacious.
This is why I wouldn't dismiss the idea.
If consciousness-induced collapse is real, it might be experimentally distinguishable from ordinary quantum mechanics or from objective-collapse models.
You would need a theory predicting something like:
f(\text{degree/type of consciousness})
]
rather than simply:
f(\text{mass},\text{size},\text{time},\ldots).
]
Then you could ask whether quantum interference survives differently depending on whether the relevant system is consciously observed.
If you found a reproducible dependence on consciousness after controlling for all physical interactions, that would be enormously important.
It would simultaneously challenge conventional physicalism and provide empirical support for a consciousness-involving ontology.
But—and this is crucial—we don't currently have such evidence.
I think your intuition is right about something important:
If consciousness is genuinely fundamental, and if we want that claim to have explanatory/causal significance rather than being an epiphenomenal add-on, consciousness-induced quantum collapse is one of the most compelling places to look.
I would not call it the best established theory, because the empirical case isn't there and the mechanism isn't sufficiently specified.
But as a research strategy, I find it substantially more interesting than simply saying:
“Consciousness is the intrinsic nature of matter, but it never changes anything.”
And this brings us to what I think is the really decisive question:
Can one formulate a consciousness-collapse theory that reproduces ordinary quantum mechanics—including the Born rule—and makes a genuinely testable prediction distinguishing it from GRW, Everett, Bohm, and standard quantum mechanics?
If the answer were yes, then we'd have moved from metaphysical
idealism inspired by quantum mechanics to an actual empirical
theory of idealism. That's the point at which
physicists would have to take it much more seriously.
Yes. I think that is a legitimate argument—but it has to be formulated more carefully than “physicists haven't agreed, therefore idealism wins.” The lack of consensus is evidence of an unresolved foundational problem, not evidence for any particular solution.
And the current state of the field actually supports the first part of your premise quite strongly. A 2025 Nature survey found substantial disagreement among quantum physicists about what quantum mechanics says about reality, and a recent review likewise describes the measurement problem as unresolved. (Nature) The Stanford Encyclopedia of Philosophy explicitly says there is no consensus among physicists or philosophers about what the empirical success of quantum theory tells us about the physical world. (Stanford Encyclopedia of Philosophy)
But there is a stronger argument available.
The measurement problem can be stated very cleanly.
Suppose the wavefunction is complete and always evolves according to the Schrödinger equation.
Then a measurement produces something like
[
a|0\rangle|{\rm apparatus\ says\ 0}\rangle
+
b|1\rangle|{\rm apparatus\ says\ 1}\rangle.
]
But we experience a definite result.
As Bell famously put the dilemma:
either the wavefunction isn't everything, or it isn't right.
The standard approaches essentially divide according to which horn they take. The SEP classifies them into:
Incomplete wavefunction: add hidden variables or other ontology.
Modified dynamics: genuine physical collapse.
Universal unitary evolution: many-worlds-type approaches.
Operational/epistemic approaches: don't regard the wavefunction as a complete physical description in the relevant sense. (Stanford Encyclopedia of Philosophy)
So your argument can begin with something quite strong:
The empirical success of quantum mechanics is not in dispute; what is unsettled is what reality must be like for that formalism to describe it.
That is a very defensible claim.
This is where the argument becomes interesting.
The wavefunction never collapses.
Instead, the different terms correspond to branches.
The attraction is obvious: keep the Schrödinger equation exactly as it is.
But the price is substantial.
You have to explain:
what the branches actually are;
why we experience one branch;
why probabilities obey the Born rule;
why the enormous superposition described by the universal wavefunction corresponds to the apparently classical world.
There is still considerable disagreement about whether Everett genuinely solves the measurement problem or relocates it into questions about probability and emergent classicality.
A physicist can reasonably reply:
“It is internally coherent and doesn't need new physics.”
That's a serious advantage.
So you shouldn't claim that Everett is simply refuted.
Bohm adds definite particle positions guided by the wavefunction.
So the wavefunction isn't the whole physical story.
You recover definite outcomes without physical collapse.
But the price is:
explicit nonlocality;
a somewhat unusual ontology;
questions about relativistic/QFT formulation;
and the fact that the wavefunction remains a central dynamical entity.
Again, this isn't a failed theory experimentally. It reproduces ordinary quantum predictions in its standard form.
So the appropriate criticism is not:
“Bohm is experimentally falsified.”
It's:
“Bohm requires substantial additional ontology and has unresolved conceptual/technical costs.”
That's much harder to dismiss.
This is actually the most interesting competitor to consciousness-collapse.
GRW/CSL-type theories say:
The Schrödinger equation is not quite right. Physical systems spontaneously collapse.
This has a major philosophical virtue:
It takes the measurement problem literally and modifies the dynamics.
No observer is required.
But now physics has acquired new parameters and new dynamics.
And crucially, objective-collapse theories are experimentally distinguishable in principle from ordinary quantum mechanics. That is their virtue as physical theories.
So here the idealist has a very interesting response:
“Why should we prefer an unexplained physical collapse mechanism over a consciousness-related collapse mechanism?”
That's a legitimate question.
But the physicist replies:
“Because objective collapse makes a physical law that can be tested, whereas consciousness-collapse currently lacks a sufficiently precise law.”
And that's a very strong response.
This one needs special treatment.
Decoherence is extraordinarily successful at explaining why interference between macroscopically distinct states becomes effectively inaccessible and why certain stable classical structures emerge.
But decoherence by itself does not select one unique outcome.
The SEP explicitly distinguishes decoherence from a complete solution to the measurement problem. (Stanford Encyclopedia of Philosophy)
A recent review similarly says that although we now understand environmental decoherence and basis selection extremely well, we still lack a fundamental explanation of why only one outcome occurs. (Taylor & Francis Online)
So if someone says:
“Decoherence solved the measurement problem.”
you have a good response:
It solved an important part of the problem, but not necessarily the problem of definite outcomes.
That's an important distinction.
Here things get more complicated.
Some versions essentially say:
Don't ask for a microscopic ontology that quantum mechanics isn't designed to provide.
The quantum/classical distinction is part of the conceptual framework.
This can be pragmatically successful.
But the idealist can ask:
Why does the classical world get to be treated as fundamentally given while the quantum world doesn't?
And more importantly:
What exactly constitutes a measurement?
If "measurement" is a primitive concept, then you've arguably put into the foundations precisely the phenomenon you're supposed to explain.
That is why many philosophers of physics regard Copenhagen as a family of views rather than a uniquely defined solution.
You can make a surprisingly powerful table:
| Approach | What it sacrifices |
|---|---|
| Everett | Single-outcome ontology / intuitive probability |
| Bohm | Locality + simple ontology |
| GRW/CSL | Exact standard quantum dynamics |
| Copenhagen | Clear universal ontology |
| QBism | Observer-independent quantum state realism |
| Relational QM | Absolute observer-independent properties |
| Decoherence alone | Doesn't by itself select one outcome |
This doesn't prove that all of them fail.
But it demonstrates something important:
There is no interpretation that simultaneously gives us an intuitively satisfactory ontology, ordinary single outcomes, exact unitary dynamics, locality, and a straightforward account of probability.
That's a much stronger proposition than “physicists disagree.”
Now imagine introducing:
[
\boxed{\text{fundamental consciousness}}
]
and saying:
The quantum state describes objective possibilities, while conscious experience is what turns a possibility into an actual experienced outcome.
Then you potentially get:
[
\text{quantum possibilities}
\rightarrow
\text{conscious experience}
\rightarrow
\text{definite physical history}.
]
This has a major conceptual advantage:
The thing that is most mysterious in the measurement problem—why there is an actual experienced outcome—is precisely consciousness.
That is not a random addition.
It connects two otherwise unexplained facts:
quantum theory produces a superposition of possible outcomes;
experience presents one definite outcome.
That's a genuinely interesting explanatory coincidence.
The physicist can say:
“You're exploiting an explanatory gap and filling it with consciousness.”
And that's fair.
The fact that both consciousness and measurement are mysterious doesn't establish that they're the same phenomenon.
There is an important logical distinction:
[
\text{Measurement problem unresolved}
]
doesn't imply
[
\text{Consciousness causes collapse}.
]
You need an additional argument.
I would formulate it as an inference to the best explanation, rather than a deduction.
Something like:
Quantum mechanics has an extraordinarily successful common predictive core.
That core does not uniquely determine an ontology.
Every major realist strategy for obtaining a definite classical world from the quantum formalism requires significant conceptual or ontological commitments.
The measurement problem specifically concerns the transition from quantum possibilities to definite experienced outcomes.
Consciousness is the one phenomenon we know to involve definite experience from the first-person perspective.
A consciousness-collapse theory can potentially connect these two otherwise independent mysteries with a single principle.
It is rationally permissible to regard consciousness-induced collapse as a serious candidate for the missing physics, rather than dismissing it as an ad hoc violation of physicalism.
That's a respectable philosophical argument.
Notice the word candidate.
That's important.
This is where I think your intuition is strongest.
A physicalist sometimes presents the alternatives as:
“We have physics. Consciousness must somehow emerge from it.”
But that's not actually an established solution to the measurement problem.
If the physicalist says:
“Everett solves everything.”
you can ask:
Does it actually explain why I experience one outcome?
If they say:
“Decoherence.”
you can ask:
Why does decoherence select one actual outcome rather than merely suppress interference?
If they say:
“GRW.”
you can ask:
Why should we modify the physical laws in precisely that way?
If they say:
“Bohm.”
you can ask:
Why should those additional hidden variables exist, and how does the ontology generalize to QFT?
If they say:
“Copenhagen.”
you can ask:
What exactly is a measurement, and why does the classical/quantum boundary exist?
None of those questions is necessarily fatal.
But the physicalist doesn't get to claim that the measurement problem has been cleanly solved merely because one of these programs exists.
Carroll's challenge to Goff was essentially:
If consciousness is fundamental, tell me what it does to the laws of physics.
Your response can be:
“Before demanding that consciousness modify the laws, why should I accept that the existing physical laws plus a physical ontology have already given us a satisfactory account of definite reality?”
That's a very important reversal.
Carroll can still respond:
“Because Everett/Bohm/GRW/etc. provide viable alternatives.”
But now the debate is no longer:
Physics vs. metaphysics.
It's:
Which foundational ontology gives the best explanation of the quantum world?
And consciousness-collapse becomes a legitimate member of that competition.
I wouldn't say:
“Because none of the physicalist interpretations has consensus, idealism is probably true.”
That's an argument from ignorance.
The correct inference is weaker:
“Because the physical ontology underlying quantum mechanics remains unsettled, physicalism does not have the privileged evidential status that one might assume. Therefore nonphysicalist ontologies deserve serious consideration.”
And then you need positive reasons for idealism.
Those reasons can come from the hard problem of consciousness, the epistemic primacy of experience, and perhaps quantum measurement.
That's a much stronger cumulative case.
You could construct the case for consciousness-collapse idealism like this:
Physicalism has not explained why physical processes are accompanied by subjective experience.
Quantum theory has not yielded a universally accepted account of why quantum possibilities correspond to one definite experienced outcome.
Perhaps these aren't two unrelated mysteries.
Perhaps:
[
\boxed{\text{consciousness}}
]
is fundamental precisely because it is involved in the transition:
[
\boxed{\text{quantum possibility}}
\rightarrow
\boxed{\text{actual experience}}
\rightarrow
\boxed{\text{classical reality}}.
]
That is a much more interesting hypothesis than either “consciousness magically emerges from matter” or “consciousness exists but does nothing.”
But it remains a hypothesis.
And the decisive next question is exactly the one we were approaching in the previous exchange:
Can we formulate consciousness-induced collapse mathematically in a way that reproduces all existing quantum results and produces new, testable predictions?
If we can, then the argument becomes dramatically stronger. If we can't, then a physicist is justified in saying that we've found an attractive metaphysical story, but not yet a physical theory.
And importantly, the absence of consensus is not merely historical: a 2025 survey of the quantum-foundations community found persistent disagreement, with no consensual interpretation emerging even as the mathematical formalism approaches its centenary. (Springer)
So I think your proposed line of attack is legitimate—but its strongest form is not “the physicists haven't solved it, therefore consciousness.” It is:
“The physicalist ontology has not earned the right to be treated as the settled background against which consciousness must somehow fit. The measurement problem shows that even the physical ontology itself remains open. Consciousness-induced collapse is therefore worth considering as a candidate foundational ontology, provided we can turn it into a quantitative theory.”
That, I think, is a position a serious philosopher of physics
could defend without overclaiming.
Exactly. You've identified a very deep problem with the whole “consciousness causes collapse” route.
If we formulate the consciousness-collapse hypothesis as an ordinary mathematical law that takes only physically measurable variables as inputs, then consciousness risks disappearing from the theory altogether.
Let's make that precise.
\mathcal L_{\rm QM}(\rho)
+
\mathcal C(\rho)
]
where (\mathcal C) is a consciousness-dependent collapse term.
If we can determine (\mathcal C) entirely from physical properties—say:
[
\mathcal C=f(\text{mass, energy, neural activity, information
integration, etc.})
]
then a sufficiently powerful computer could calculate the collapse probabilities without ever being conscious.
At that point a physicist can say:
“You haven't introduced consciousness into physics. You've introduced a new physical collapse law correlated with what we happen to call consciousness.”
And yes, the metaphysical distinction from GRW-like objective collapse becomes very thin.
This exposes a distinction that is easy to miss.
For example:
[
P(\text{collapse})=f(C)
]
where (C) is some variable representing consciousness.
But then we have to ask:
What physically measurable quantity is (C)?
If (C) can be completely reduced to neural activity, information integration, computational complexity, etc., then we've essentially reduced it to physics.
If it can't, then we have a new primitive variable.
But then the crucial question becomes:
How could physics measure (C)?
That's the hard part.
Suppose instead that there really is a primitive quantity:
[
C
]
which represents phenomenal experience itself.
And suppose:
[
\text{physical state} + C
\rightarrow
\text{collapse probability}.
]
Now we genuinely have something different from objective collapse.
A computer simulation of the physical variables would not be sufficient to determine the complete evolution unless the computer also instantiated the relevant consciousness.
That's a genuine ontological difference.
But we've paid a very large price.
We now need a theory of what (C) is and how it enters physics.
And that's precisely the problem that Carroll was pushing Goff toward.
I think you've uncovered something important:
[
C=f(P)
]
Then consciousness-induced collapse becomes:
[
\text{collapse}=g(P)
]
and it is effectively an objective physical collapse theory.
Idealism has disappeared.
[
C\neq f(P)
]
and:
[
\text{collapse}=g(P,C).
]
Now consciousness genuinely makes a causal difference.
But we've introduced a new ontological primitive and need psychophysical laws.
This is no longer ordinary physicalism—but it's arguably dual-aspect monism, panpsychism, or interactionist property dualism more naturally than idealism.
This is the genuinely idealist possibility.
Instead of:
[
P + C \rightarrow P'
]
you have something more like:
[
C \rightarrow \text{physical description }P.
]
The physical variables themselves are derived from or constituted by the underlying experiential reality.
Then asking for a consciousness variable (C) to be inserted into the Schrödinger equation may actually be asking the wrong question.
In Kastrup's ontology, consciousness isn't supposed to be a thing inside the physical universe that interacts with other things.
That's basically dualism.
Instead:
The physical universe is an appearance within consciousness.
So the idealist isn't saying:
[
\text{matter} + \text{consciousness} \rightarrow \text{matter}.
]
They're saying:
[
\text{consciousness}
\rightarrow
\text{the entire physical domain}.
]
This means the ordinary causal laws of physics describe regularities within the appearance.
You don't need to add a “consciousness term” to the Schrödinger equation any more than you need to add “redness” as a new force to the equations describing a red object.
And this is where I think your objection goes even deeper.
Suppose an idealist says:
“The physical world is constituted by consciousness.”
The physicist asks:
“Fine. Can I calculate what happens?”
The idealist says:
“Yes. Use quantum mechanics.”
Then the physicist asks:
“Does the calculation depend on whether anyone is actually conscious?”
If the answer is no, then consciousness appears explanatorily redundant.
If the answer is yes, we need a new law.
And if the answer is:
“The physical laws themselves are expressions of the intrinsic dynamics of consciousness,”
the physicist can ask:
“What does that mean mathematically?”
And we're back at the original problem.
Consciousness is the fundamental substance/ontology.
This doesn't necessarily imply that consciousness modifies physical laws.
Consciousness actively determines physical events.
This is much closer to consciousness-collapse theories.
And the two aren't equivalent.
Kastrup is primarily defending the first.
Wigner/Stapp-style approaches are closer to the second.
Consider a computer simulation.
Suppose I simulate the entire universe on a computer.
The simulation contains:
particles,
fields,
quantum states,
brains,
observers.
The simulation evolves according to exactly the same equations.
A physicalist says:
“If the simulation contains the right physical structure, it contains consciousness.”
An idealist can instead say:
“The simulation describes the structure of possible experiences, but the equations themselves don't tell you whether there is actual experience.”
This is closely related to the zombie argument.
Two worlds could be physically identical:
[
P_1=P_2
]
while allegedly differing in:
[
C_1\neq C_2.
]
If that's coherent, then consciousness isn't reducible to the physical description.
But—and this is crucial—this doesn't give consciousness causal power.
So we have a very strange possibility:
consciousness is ontologically fundamental but causally invisible.
That's exactly what Carroll finds unsatisfying.
Perhaps causal efficacy is the wrong criterion for fundamentality.
Consider a mathematical example.
Suppose the fundamental object is a geometrical structure.
You don't ask:
“What extra force does geometry exert?”
Geometry isn't a force. It is part of the description of what the physical system is.
Similarly, the idealist could say:
Consciousness isn't a force acting on physical events. It is the intrinsic reality of which physical events are the structural description.
Then the fact that a computer can simulate the physical structure doesn't show that it has simulated consciousness.
It has simulated the relations.
Whether the simulation actually feels like anything is another question.
This is essentially the Russellian move.
The crucial question becomes:
Can two systems have exactly the same physical/functional structure while only one has consciousness?
If yes, then physical description doesn't exhaust reality, and something like Russellian monism or idealism remains possible.
If no, then consciousness is determined by physical structure, and physicalism becomes much more plausible.
And notice how this question is not really a quantum-mechanical question anymore.
Quantum mechanics may tell us that the physical structure is strange, relational, probabilistic, etc.
But it doesn't by itself answer whether:
[
P \rightarrow C
]
is a necessary identity.
I don't think the strongest case for idealism is actually:
“Consciousness collapses the wavefunction.”
That gives consciousness causal efficacy, but it risks making consciousness just another variable in an expanded physical theory.
The more profound idealist claim is:
“The physical description itself is not ontologically exhaustive.”
Then quantum mechanics becomes relevant because it gives us reasons to question the classical idea that physics simply reveals a world of self-subsistent objects with intrinsic properties.
But quantum mechanics does not by itself establish what the missing ontology is.
Imagine we eventually develop a perfect equation:
[
P(\text{outcome}) =
F(\text{physical quantum state})
]
that reproduces every result attributed to consciousness-induced collapse.
Then indeed:
Why call it consciousness-induced collapse?
It would just be objective collapse.
To preserve a genuinely consciousness-dependent theory, you would need:
[
P(\text{outcome}) =
F(\text{physical state},\text{phenomenal state})
]
where the phenomenal state is not reducible to the physical state.
But then you have the interaction problem.
So consciousness-collapse theory faces a very sharp dilemma:
[
\boxed{
\begin{array}{c}
\text{Consciousness reducible to physical variables}\
\Downarrow\
\text{objective collapse}
\end{array}
}
]
versus
[
\boxed{
\begin{array}{c}
\text{Consciousness irreducible}\
\Downarrow\
\text{new psychophysical law required}
\end{array}
}
]
And neither branch gives you Kastrup-style idealism automatically.
That's why I would now distinguish three projects much more sharply:
Consciousness-collapse:
tries to make consciousness causally efficacious.
Russellian monism:
tries to make consciousness the intrinsic nature behind physical
structure.
Idealism:
tries to make consciousness the fundamental ontology from which
the physical description itself derives.
Your objection shows why the third cannot simply be reduced to the first.
And, interestingly, I think this is where the strongest version
of the idealist argument becomes less dependent on
quantum mechanics, not more. Quantum theory can undermine naïve
physical realism, but the decisive argument for idealism has to
concern the relationship between phenomenal reality and
physical description itself.
Yes—but not quite in the form of a systematic survey of all interpretations, and there is an important distinction between what Kastrup himself has written and material published on his site by others.
I found several relevant pieces.
Kastrup wrote an essay called “Thinking Outside the Quantum Box”, where he explicitly discusses the interpretive problem in quantum mechanics and uses it in support of his idealist position. He begins by noting that there are numerous competing interpretations and focuses particularly on the implications of Bell-type nonlocality for the idea that particles are independently existing objects. (Science and Nonduality (SAND))
This is probably the closest thing to what you're looking for, although it is not a detailed technical comparison of Copenhagen vs. Everett vs. Bohm vs. GRW vs. QBism, etc.
His basic strategy is:
quantum mechanics makes the naïve idea of independently existing physical objects problematic → therefore we should reconsider what “physical reality” means → idealism becomes a plausible alternative.
That is very relevant to our discussion, but it doesn't really address Carroll's challenge about whether consciousness has causal efficacy.
Kastrup has an older essay explicitly titled “My philosophy and quantum physics.” He explains how his idealism relates to quantum mechanics, while making an interesting point that is relevant to our previous discussion: his idealism doesn't actually require quantum mechanics to imply that unobserved physical objects cease to exist.
He says that his worldview is compatible even with a classical picture in which objects have definite properties when nobody is observing them, provided those objects are understood as experiences within a non-personal consciousness rather than as things existing outside consciousness. (bernardokastrup.com)
That's quite revealing.
It means Kastrup's idealism is not fundamentally a “consciousness collapses the wavefunction” theory. Quantum mechanics is supporting evidence for his metaphysics, not its foundation.
This is probably the most academically useful thing I found.
Kastrup published:
“Reasonable Inferences From Quantum Mechanics: A Response to ‘Quantum Misuse in Psychic Literature’”
in the Journal of Near-Death Studies in 2019.
The paper explicitly discusses several foundational issues in quantum mechanics, including physical realism and nonlocality. Its purpose is partly defensive—he argues that some criticisms of popular uses of quantum mechanics go too far—but it contains Kastrup's own assessment of what can legitimately be inferred from quantum foundations. (PhilArchive)
So if you're looking for Kastrup engaging quantum mechanics as an epistemological/ontological issue rather than merely mentioning it, this is worth reading.
This becomes especially interesting.
In a 2023 discussion concerning Tim Maudlin, Kastrup criticized Many-Worlds and Bohmian mechanics. He argued, among other things, that Everett's “everything happens” structure undermines its explanatory/predictive usefulness, and he criticized Bohmian mechanics on theoretical grounds. (bernardokastrup.com)
I would be cautious about taking his characterization of Bohmian mechanics at face value, though. That's Kastrup's philosophical criticism, not a consensus assessment among physicists.
I searched specifically for something along the lines of:
Kastrup: systematic critical comparison of Copenhagen, Many Worlds, Bohm, GRW, QBism, relational QM, etc., with an argument that idealism is superior.
I don't find evidence that he has published such a comprehensive treatment.
His writings tend to follow the opposite direction:
[
\text{problems with physical realism}
\rightarrow
\text{idealism}
]
and then:
[
\text{quantum mechanics}
\rightarrow
\text{additional motivation for questioning physical realism}.
]
He doesn't seem to make:
[
\text{systematically compare all interpretations}
\rightarrow
\text{eliminate them}
\rightarrow
\text{idealism}.
]
And given our preceding discussion, that is precisely the gap in his argument that I think you're noticing.
Kastrup's bibliography on PhilPapers explicitly places his work under categories including:
Consciousness and the Interpretation of Quantum Mechanics
Interpretations of Quantum Mechanics
Relational Interpretations
Quantum Nonlocality
Physical Realism / Anti-Realism
and his 2017 paper On the Plausibility of Idealism: Refuting Criticisms is directly concerned with distinguishing idealism from physicalism and panpsychism. (PhilPapers)
So there is definitely a body of work there, but it is fragmented across philosophical papers, essays, and responses rather than presented as a systematic philosophy-of-quantum-mechanics monograph.
After our discussion, I'm actually more interested in what Kastrup does with the measurement problem than I was initially.
Because there seems to be a potentially important tension:
Kastrup's idealism doesn't require consciousness to cause wavefunction collapse.
But then Carroll's objection arises:
If consciousness doesn't modify physical dynamics, in what sense does its fundamentality explain anything about physical reality?
And if Kastrup does invoke consciousness to explain collapse, your subsequent objection arises:
If the consciousness-dependent collapse can be represented by an ordinary mathematical law and simulated without consciousness, what remains specifically idealist about it?
That is a much deeper problem than simply “does quantum mechanics support idealism?”
I don't recall Kastrup having given a completely satisfactory answer to that particular dilemma. His idealism is primarily an ontological thesis about what the physical world is, rather than a proposed modification of quantum dynamics.
If you want, I can do something more useful than just list his
writings: I can reconstruct Kastrup's actual position on
quantum mechanics from his papers/books, then compare it
point-by-point with Carroll's objection, Goff's panpsychism,
Stapp's consciousness-collapse proposal, QBism, Bohm, Everett,
and objective-collapse theories. That would tell us
whether Kastrup's position actually survives the objection you've
been developing.
Yes—and this exposes an ambiguity in the word “physicalist.” A mathematical law does not by itself make a theory physicalist.
I overstated that point earlier.
The real issue is what the mathematical law is a law of and what ontology it presupposes.
Suppose an idealist says:
[
P(\text{physical outcome})=F(C,\text{quantum state})
]
where (C) is phenomenal consciousness.
Giving (F) mathematically does not turn the theory into physicalism. It could instead be a theory saying:
There is a fundamentally mental reality, and it obeys mathematically precise psychophysical laws.
That is perfectly compatible with substance idealism.
The important question is whether (C) can be eliminated:
[
F(C,P)=G(P).
]
If yes, then consciousness is not doing fundamental explanatory work in the theory.
If no—if the phenomenal variable is irreducible—then the theory remains non-physicalist even though it is mathematically formulated.
Because there are really two different standards being applied.
You can coherently say:
“Consciousness is fundamental and physical reality is an appearance of it.”
That is a metaphysical position.
But it may not tell us why the appearance has precisely the structure we observe.
A physicist asks:
“If your ontology is true, what observable consequences follow?”
That's completely reasonable.
If idealism makes no possible difference to anything we could observe, then physics cannot adjudicate between it and physicalism.
So asking for mathematical articulation isn't asking:
“Prove that idealism is physicalism.”
It's asking:
“Show me where your metaphysics enters the empirical world.”
Those are very different requests.
Suppose you formulate an idealist theory in which consciousness is fundamental but doesn't alter any physical predictions.
Then:
P(\text{observations}|\text{physicalism}).
]
The two theories are empirically equivalent.
Idealism may still be philosophically preferable because it offers a better account of consciousness, but physics cannot choose between them.
That's not necessarily a defect. Lots of metaphysical claims aren't directly testable.
But if an idealist says:
“Quantum mechanics provides scientific evidence for idealism,”
then the standard becomes much higher.
The idealist now owes us some account of why quantum mechanics follows from the idealist ontology or why it favors it over competing ontologies.
Consider:
[
\boxed{\text{consciousness}}
]
as fundamental, with a psychophysical law
[
\boxed{P(\text{outcome})=F(C,\psi)}.
]
This is not physicalism.
It's more like:
Idealist ontology + physical/psychophysical dynamics.
The mathematical law tells us how the mental and physical descriptions are related.
That's no more “physicalist” merely because it has equations than Newton's laws are “materialist” merely because they have equations.
The danger is that the supposedly phenomenal variable (C) turns out to be just a proxy for a physical variable.
For example:
[
C = f(\text{neural complexity}).
]
Then:
[
P(\text{collapse})=F(f(P),P)
]
can be rewritten as:
[
P(\text{collapse})=G(P).
]
Now consciousness has disappeared from the fundamental theory.
That's what I meant earlier by the computer-simulation problem.
But the mere existence of a mathematical law isn't the problem.
The problem is reducibility of the variables appearing in the law.
An idealist could say something more radical:
“You are asking me to put consciousness into the equations as a variable, but consciousness is not an entity inside the physical system. It is the fundamental reality of which the physical system is a representation.”
Then perhaps the relationship is not:
[
C + P \rightarrow P'
]
at all.
It might instead be:
[
\boxed{\text{fundamental experiential dynamics}}
]
[
\Downarrow
]
[
\boxed{\text{physical laws as a structural description}}
]
In that case, the idealist's scientific task would be not necessarily to add a consciousness term to quantum mechanics, but to explain why the physical formalism is the correct mathematical representation of the underlying mental dynamics.
That's a much more ambitious project.
Russellian monism can make this move particularly naturally.
Suppose fundamental reality (X) has:
structural properties (S), described by physics;
phenomenal/intrinsic properties (C).
Then:
[
X \xrightarrow{\text{physics}} S
]
and
[
X \xrightarrow{\text{first-person}} C.
]
You don't necessarily need a new equation saying that (C) pushes (S) around.
The two are aspects of the same underlying state.
A mathematical theory could describe the structural dynamics perfectly while leaving the intrinsic nature unspecified.
That's still non-physicalist if the intrinsic nature is genuinely irreducible.
Not:
“If you give idealism a mathematical law, you've made it physicalist.”
That's false.
Rather:
“If all the variables in your idealist law can ultimately be specified in purely physical terms, then the theory may collapse into physicalism.”
And conversely:
“If the theory contains genuinely irreducible phenomenal variables, mathematical articulation does not make it physicalist.”
This is why requesting a mathematical formulation of a non-physicalist theory is perfectly legitimate.
In fact, it's exactly what you want if you want to distinguish a non-physicalist theory from a purely metaphysical reinterpretation of physics.
Carroll says, essentially:
“If consciousness is fundamental, tell me what it does to the physics.”
The idealist can answer:
“Perhaps nothing. Fundamentality doesn't entail an additional causal force. But if you want me to claim that consciousness explains an empirical feature of physics, then yes, I owe you a precise account of the relationship between phenomenal reality and physical dynamics.”
And if the idealist does propose a consciousness-dependent collapse law, Carroll can then ask whether the phenomenal variable is reducible to physics.
So there are actually two independent questions:
Is consciousness fundamental?
Does consciousness have an empirically detectable causal role in physical dynamics?
An idealist can answer yes/no, yes/yes, or even yes/unknown to those two questions.
And importantly, “yes” to the first does not logically require “yes” to the second.
That distinction is probably the key to untangling the whole
disagreement we've been discussing.
Yes—if by “simulate consciousness” you mean that the simulation itself would thereby be conscious, then your objection is very strong. But there is a crucial distinction between simulating the causal consequences of consciousness and simulating consciousness itself.
That's exactly where the issue becomes philosophically subtle.
Suppose an idealist theory says:
[
C \rightarrow P
]
where (C) is phenomenal consciousness and (P) is physical behavior.
Now suppose we discover a function
[
P=f(C).
]
A computer can calculate (f(C)), but that doesn't yet mean the computer possesses (C).
For example, a computer can calculate:
[
f(\text{temperature}) = \text{pressure}
]
without itself being hot.
So computability of the causal relationship doesn't imply instantiation of the entity represented by the variable.
But your objection becomes much stronger if the law is instead something like:
[
P(t+\Delta t)=F(P(t))
]
where the physical state alone is sufficient to generate every consequence that the theory attributed to consciousness.
Then yes:
The consciousness has become explanatorily dispensable.
And that's a serious problem for a consciousness-collapse theory.
Suppose I write a perfect computer simulation of a hurricane.
The simulation contains:
simulated air,
simulated water,
simulated pressure,
simulated wind.
It can reproduce the hurricane's dynamics.
But there isn't necessarily a real hurricane inside the computer.
Likewise, an idealist could say:
A classical computer can simulate the physical consequences associated with conscious experience without having the experience itself.
That is perfectly compatible with non-physicalism.
The computer is simulating:
[
\text{the structure/behavior associated with } C
]
rather than instantiating:
[
C.
]
This is actually one of the central issues in the philosophy of consciousness: whether functional/structural equivalence is sufficient for phenomenal equivalence.
Suppose the proposed law is:
[
P(\text{collapse}) =
F(\text{neural information integration}).
]
Now I build a classical computer that perfectly reproduces the relevant information-processing structure.
The computer predicts:
[
P(\text{collapse})=0.73.
]
But the idealist says:
“The computer isn't conscious, so the actual collapse probability isn't 0.73.”
Then we have a problem:
If there isn't one, the theory is not physically complete.
We need something like:
[
P(\text{collapse})=F(P,C)
]
where (C) is a genuinely irreducible phenomenal variable.
Now the computer cannot determine the collapse merely from its physical simulation.
That preserves the non-physicalism.
But it creates the interaction problem we discussed earlier.
You cannot simultaneously have all three of these:
[
C\neq f(P)
]
[
P(t+\Delta t)=F(P(t))
]
[
C\rightarrow P(t+\Delta t)
]
because 2 and 3 together imply that (C) isn't actually needed to determine the physical evolution.
So if an idealist wants causally efficacious consciousness, they have to give up something like physical causal closure.
That's precisely Carroll's challenge.
The idealist could reject the assumption that a classical simulation is ontologically equivalent to the system being simulated.
Suppose a brain has conscious experience because its physical processes are the appearance of underlying experiential dynamics.
Then:
[
\text{brain}
]
and
[
\text{computer simulation of brain}
]
could be structurally identical while differing ontologically.
The computer reproduces:
[
P
]
but doesn't necessarily instantiate:
[
C.
]
This is entirely coherent under idealism.
But notice what happens:
The computer can reproduce all the physical predictions without reproducing consciousness.
Therefore, consciousness again doesn't need to affect the physical predictions.
And we've returned to the problem you identified earlier.
For a non-physicalist consciousness-collapse theory, we seem to want:
[
\boxed{
\begin{array}{c}
\text{Consciousness is irreducible}\
\text{AND}\
\text{consciousness causally affects physics}\
\text{AND}\
\text{the causal effect cannot be reduced to physical variables}
\end{array}}
]
But then the theory must contain a genuinely non-physical input into physical dynamics.
That isn't impossible.
It's essentially a form of interactionist dualism or psychophysical interactionism.
But it isn't straightforward idealism anymore.
If the goal is specifically idealism, then trying to make consciousness a causal variable inside quantum mechanics may actually be the wrong strategy.
Because the moment you say:
[
\text{physical state} + \text{consciousness}
\rightarrow
\text{physical state},
]
you have conceptually divided reality into:
a physical domain;
a mental domain;
an interaction law between them.
That's much closer to dualism than to idealism.
Idealism wants instead:
[
\boxed{\text{experience is fundamental}}
]
and
[
\boxed{\text{physicality is derivative}}.
]
Then the computer simulation problem doesn't threaten the ontology.
A computer can simulate the physical appearance without necessarily reproducing the experiential reality.
If idealism doesn't require consciousness to alter physical dynamics, quantum mechanics may not provide an empirical route to distinguishing idealism from physicalism at all.
You could have:
Physicalism
[
P\rightarrow C
]
and
Idealism
[
C\rightarrow P_{\text{appearance}}
]
while both generate exactly the same:
[
P(t)
]
for every possible experiment.
Then the difference is metaphysical rather than physical.
And I think this is the strongest challenge to the quantum-idealism program.
Your argument doesn't show that mathematical articulation makes a theory physicalist.
It shows something more interesting:
If a non-physicalist theory claims that consciousness has a causal role in producing physical events, then a mathematical formulation must preserve an irreducible phenomenal variable. Otherwise the theory collapses into an ordinary physical theory.
And if it does preserve such a variable, we've created a new psychophysical interaction law whose interpretation is no longer straightforwardly idealist.
So the most coherent idealism may actually be one that doesn't try to make consciousness a causal variable at all.
That's a much more radical position: consciousness isn't something that acts upon the physical world.
The physical world is what a certain aspect/structure of consciousness looks like.
And that, rather than consciousness-induced collapse, is where I think Kastrup's position is philosophically strongest—and also where it becomes hardest to turn into an empirical scientific theory.