1. First foundations of mathematics

1.1. Introduction to the foundations of mathematics

Mathematics and theories

Mathematics is the study of systems of elementary objects, whose only nature is to be exact, unambiguous (two objects are equal or different, related or not; an operation gives an exact result...). Such systems are conceived independently of our usual world, even if many of them can resemble (thus be used to describe) diverse aspects of it. Mathematics as a whole can be seen as «the science of all possible worlds» of this kind (of exact objects).
Mathematics is split into diverse branches, implicit or explicit frameworks of any mathematical work, that may be formalized as (axiomatic) theories. Each theory is the study of a supposedly fixed system (world) of objects, called its model. But each model of a theory may be just one of its possible interpretations, among other equally legitimate models. For example, roughly speaking, all sheets of paper are systems of material points, models of the same theory of Euclidean plane geometry, but independent of each other.

Foundations and developments

Each theory starts with a foundation, that is the data of a list of pieces of description specifying what it knows or assumes of its model(s) (its kind or shape). This includes a list of formulas (statements) called axioms, expressing the required properties of models, i.e. selecting its accepted models as the systems where the axioms are true, from the whole range of possible systems where they can be interpreted.
Then, the study of a theory progresses by choosing some of its possible developments : new concepts and information about its models, resulting from its given foundation, and that we can add to it to form its next foundation.
In particular, a theorem of a theory, is a formula deduced from its axioms, so that it is known as true in all its models. Theorems can be added to the list of axioms of a theory without modifying its meaning.

Other possible developments (not yet chosen) can still be operated later, as the part of the foundation that could generate them is preserved. Thus, the totality of possible developments of a theory, independent of the order chosen to process them, already forms a kind of «reality» that these developments explore (before the Completeness theorem will finally show how the range of possible theorems precisely reflects the more interesting reality of the diversity of possible models).

There are possible hierarchies between theories, where some can play a foundational role for others. For instance, the foundations of several theories may have a common part forming a simpler theory, whose developments are applicable to all.
A fundamental work is to develop, from a simple initial basis, a more complete foundation endowed with efficient tools opening more direct ways to further interesting developments.

The cycle of foundations

Despite the simplicity of nature of mathematical objects, the general foundation of all mathematics turns out to be quite complex (though not as bad as a physics theory of everything). Indeed, it is itself a mathematical study, thus a branch of mathematics, called mathematical logic. Like any other branch, it is made of definitions and theorems about systems of objects. But as its object is the general form of theories and systems they may describe, it provides the general framework of all branches of mathematics... including itself.

And to provide the framework or foundation of each considered foundation (unlike ordinary mathematical works that go forward from an assumed foundation), it does not look like a precise starting point, but a sort of wide cycle composed of easier and harder steps. Still this cycle of foundations truly plays a foundational role for mathematics, providing rigorous frameworks and many useful concepts to diverse branches of mathematics (tools, inspirations and answers to diverse philosophical questions).

(This is similar to dictionaries defining each word by other words, or to another science of finite systems: computer programming. Indeed computers can be simply used, knowing what you do but not why it works; their working is based on software that was written in some language, then compiled by other software, and on the hardware and processor whose design and production were computer assisted. And this is much better than at the birth of this field.)
It is dominated by two theories: Each one is the natural framework to formalize the other: each set theory is formalized as a theory described by model theory; the latter better comes as a development from set theory (defining theories and systems as complex objects) than directly as a theory. Both connections must be considered separately: both roles of set theory, as a basis and an object of study for model theory, must be distinguished. But these formalizations will take a long work to complete, especially for this following last piece: Model theory and proof theory are essentially unique, giving a clear natural meaning to the concepts of theory, theorems and consistency of each theory.

Set theory and Foundations of mathematics
1. First foundations of mathematics
1.1. Introduction to the foundations of mathematics
1.2. Variables, sets, functions and operations
1.3. Form of theories: notions, objects, meta-objects
1.4. Structures of mathematical systems
1.5. Expressions and definable structures
1.6. Logical connectives
1.7. Classes in set theory
1.8. Binders in set theory
1.9. Quantifiers
1.10. Formalization of set theory
1.11. Set generation principle
Philosophical aspects
Time in model theory
Time in set theory
Interpretation of classes
Concepts of truth in mathematics
2. Set theory (continued) 3. Model theory
Other languages:
FR : Introduction au fondement des mathématiques