## 1.7. Classes in set theory

For any theory, a class is a unary predicate A seen as the set of objects where A is true, that is «the class of all x such that A(x)».
In particular for set theory, each set E is synonymous with the class of the x such that xE (defined by the formula xE with argument x and parameter E). However, this involves two different interpretations of the notion of set, that need to be distinguished by the following means.

#### The unified framework of theories

Attempts to formalize one-model theory in first-order logic cannot completely specify the meta-notions of «expressions» and «proofs». Indeed as will be explained in 4.7 (Non-standard models of Arithmetic), any first-order theory aiming to describe finite systems without size limit (such as expressions and proofs) inside its model (as classes included in a type), will still admit in some models some pseudo-finite ones, which are infinite systems it mistakes as «finite» though sees them larger than any size it can describe (as the latter is an infinity of properties which it cannot express as a whole to detect the contradiction ; these systems will also be called non-standard as «truly finite» will be the particular meaning of «standard» when qualifying kinds of systems which should normally be finite).
To fill this gap will require a second-order universal quantifier (1.9), whose meaning is best expressed (in appearance though not really completely) after insertion in set theory (whose concept of finiteness will be defined in 4.5). As this insertion turns its components into free variables whose values define its model [T, M], their variability removes its main difference with model theory (the other difference is that model theory can also describe theories without models). This view of model theory as developed from set theory, will be exposed in Parts 3 and 4, completing the grand tour of the foundations of mathematics after the formalization of set theory in a logical framework.

Given a theory T so described, let T0 be the external theory, also inserted in set theory, which looks like a copy of T as any component k of T0 has a copy as an object serving as a component of T. In some proper formalization, T0 can be defined from T as made of the k such that («k» ∈ T) is true, where the notation «k» abbreviates a term of set theory designating k as an object, and the truth of this formula means that the value of this term in the universe belongs to T.

This forms a convenient unified framework for describing theories interpreted in models, encompassing both previous ones (set-theoretical and model-theoretical): all works of the theory T0 (expressions, proofs and other developments), have copies as objects formally described by the model theoretical development of set theory as works of the theory T. In the same universe, any system M described as a model of T is indirectly also a (set-theoretical) model of T0.
This powerful framework is bound to the following limits :
• Starting from an arbitrary theory T0 with infinitely many components, a corresponding T cannot be directly defined, as a definition must be expressible by finite means and thus cannot depend on an infinity of meta-objects. Any infinite list of components of T0 must be defined by some rules, for getting T as defined by the same rules.
• The risk for a universe to be non-standard in the sense of containing pseudo-finite systems (which formalizations of set theory in first-order logic cannot prevent), leads to the following possible discrepances between T0 and T :
• T0 only copies the standard components of T, ignoring non-standard ones that usually exist for infinite theories in a non-standard universe. Then a model of T0 in this universe may fail to be a model of T by not fitting some non-standard components (axioms) of T.
• By the incompleteness theorem, T may be inconsistent while T0 is consistent, either due to non-standard axioms of T, or to a non-standard contradiction between standard axioms, which may exist even if these theories are identical (having a finite initial content). As such a contradiction means that T has no model in this universe (non-standard «proofs» are only valid in the universe containing them), models of T0 may either have the above failure (in the infinite case), or only exist outside this universe.
So understood, the conditions of use of this unified framework of theories, are usually accepted as legitimate assumptions, by focusing on well-described theories (though no well-described set theory can be the "ultimate" one as mentioned below), interpreted in standard universes whose existence is admitted on philosophical grounds; this will be further discussed in philosophical pages.

#### Standard universes and meta-sets

From now on, in the above unified framework, the theory T0 describing M and idealized as an object T, will be set theory itself. Taking it as an identical copy of the set theory serving as framework, amounts to taking the same set theory interpreted by two universes, that will be distinguished by giving the meta- prefix to the interpretation in the role of framework.

Aside generic interpretations, set theory has a standard kind of interpretation into itself where each set is interpreted by the class (meta-set) of its elements (the synonymous object and meta-object are now equal), and each function is interpreted by its synonymous meta-function (see more details with how it relates with finiteness). This way, any set will be a class, while any class is a meta-set of objects. But some meta-sets of objects are not classes (no formula with parameters can define them); and some classes are not sets, such as the class of all sets (see Russell's paradox in 1.8), and the universe (class of all objects, defined by 1).

A kind of theoretical difference between both uses of set theory will turn out to be irreducible (by the incompleteness theorem): for any given (invariant) formalization of set theory, the existence of a model of it (universe), or equivalently its consistency, formalized as a set theoretical statement with the meta interpretation, cannot be logically deduced (a theorem) from the same axioms. This statement, and thus also the stronger statement of the existence of a standard universe, thus forms an additional axiom of the set theory so used as framework.

#### Definiteness classes

Set theory accepts all objects as «elements» that can belong to sets and be operated by functions (to avoid endless further divisions between sets of elements, sets of sets, sets of functions, mixed sets...). This might be formalized keeping 3 types (elements, sets and functions), where each set would have a copy as element, identified by a functor from sets to elements, and the same for functions. But beyond these types, our set theory will anyway need more notions as domains of its structures, which can only be conveniently formalized as classes. So, the notions of set and function will also be classes named by predicate symbols:

Set = «is a set»
Fnc = «is a function»

In first-order logic, any expression is ensured to take a definite value, for every data of a model and values of all free variables there (by virtue of its syntactic correction, that is implicit in the concept of «expression»). But in set theory, this may still depend on the values of free variables.
So, an expression A (and any structure defined from it) will be called definite, if it actually takes a value for the given values of its free variables (seen as arguments and parameters of any structure it defines). This condition is itself an everywhere definite predicate, expressed by a formula dA with the same free variables. Choosing one of these as argument, the class it defines is the meta-domain, called class of definiteness, of the unary structure defined by A.
Expressions should be only used where they are definite, which will be done rather naturally. The definiteness condition of (xE) is Set(E). That of the function evaluator f(x) is Fnc(f) ∧ x ∈ Dom f.
But the definiteness of the last formula needs a justification, given below.

#### Extended definiteness

A theory with partially definite structures, like set theory, can be formalized (translated) as a theory with one type and everywhere definite structures, keeping intact all expressions and their values wherever they are definite : models are translated one way by giving arbitrary values to indefinite structures (e.g. a constant value), and in the way back by ignoring those values. Thus, an expression with an indefinite subexpression may be declared definite if its final value does not depend on these extra values.
In particular for any formulas A and B, we shall regard the formulas AB and AB as definite if A is false, with respective values 0 and 1, even if B is not definite. So, let us give them the same definiteness condition dA ∧ (A ⇒ dB) (breaking, for A ∧ B, the symmetry between A and B, that needs not be restored). This formula is made definite by the same rule provided that dA and dB were definite. This way, both formulas

A ∧ (BC)
(AB) ∧ C

have the same definiteness condition (dA ∧ (A ⇒ (dB ∧ (BdC)))).

Classes will be defined by everywhere definite predicates, easily expressible by the above rule as follows.
Any predicate A can be extended beyond its domain of definiteness, in the form dAA (giving 0), or dAA (giving 1).
For any class A and any unary predicate B definite in all A, the class defined by AB is called the subclass of A defined by B.

 Set theory and Foundations of Mathematics 1. First foundations of mathematics 1.7. Classes in set theory⇨1.8. Binders in set theory Philosophical aspects Time in model theory ⇨ Time between theories 2. Set theory (continued) - 3. Algebra - 4. Arithmetic 5. Second-order foundations
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FR : 1.7. Classes en théorie des ensembles