foundations

category theory

# Contents

## Overview

There are two big questions about category theory and the logical foundations of mathematics:

1. What foundations are adequate to formulate category theory?
2. How can category theory provide a foundation for mathematics?

These questions also apply to higher category theory, which also involves the relation between them. (2-categories as a foundation for categories, for example.)

## Mathematical foundations of category theory

The problem with mathematical foundations of category theory is that in category theory we frequently speak of large categories, which it is tricky to deal with rigorously in the usual sort of set theories.

One common view seems to be to found category theory on a theory of sets and classes; see the English Wikipedia's definition, for example. But the standard reference, Saunders Mac Lane's Categories for the Working Mathematician, assumes the existence of a universe (an inaccessible cardinal) instead. Both of these approaches rely on a distinction between small and large categories. There is a category of all small categories, but this category is not itself small; there is no category of all categories.

Alexander Grothendieck used more (although apparently he did not need to); he used what we now call Grothendieck universes. He assumed that every set is contained within a universe; that is, for every cardinal number $\kappa$, there is a cardinal inaccessible from $\kappa$. (This is still a rather moderate axiom, compared to some of the large-cardinal axioms studied by set theorists.) Now one has a relative notion of small and large; the category of all $U$-small categories (where $U$ is some universe) is $U$-large but must be $U'$-small for some other universe $U'$, and there exists a category (which is both $U$-large and $U'$-large) of all $U'$-small categories.

If one does not accept the axiom of choice, then there are additional complications in general category theory. In particular, one must distinguish between a universal property (for example, having all products) and having a universal structure realising that property (in the example, a functor taking each pair $(x,y)$ of objects to a specific product cone $x \leftarrow x \times y \rightarrow y$). This difficulty was overcome by Michael Makkai using anafunctors, but these have not been widely adopted, even by constructivists.

For a discussion of set theoretic foundations of category theory see the references below.

### The concept of identity

One way to think of category theory is as a framework in which the idea is formalized that every kind of equality is really secretly a choice of isomorphism or equivalence. In some sense the notion of identity potentially breaks the principle of equivalence.

Michael Makkai works on a language, FOLDS (‘first-order logic with dependent sorts’), which is designed to make it impossible to formulate any statements that break the principle of equivalence.

## Categorial foundations of mathematics

Bill Lawvere proposed to found mathematics on ETCC (for ‘ Elementary Theory of the Category of Categories’), a first-order axiomatisation of the category of categories. This has not been very successful,

but his other proposal, a first-order axiomatisation of the category of sets, works well. These and related approaches to foundations may be called structural or categorial (or categorical, which is more common but clashes with another sense of ‘categorical’ in logic).

Lawvere's system ETCS (for ‘the Elementary Theory of the Category of Sets’) essentially states that the category of sets is a topos with certain properties, in particular a well-pointed topos. This can be stated in elementary (first-order) terms; indeed, the system of axioms for ETCS in 1965 was retrospectively an important step in Lawvere’s quest to give a first-order axiomatization for the topos concept that was originally formulated in higher-order terms by the Grothendieck school, resulting in 1970 in the by the now-default notion of elementary topos that subsumes the original notion, now called Grothendieck topos, as an important special case.

It is also possible to found mathematics on the internal language of a topos. In this case, the topos need not be well-pointed (and indeed, the condition that a topos be well-pointed cannot be stated in its own internal language; or if you prefer, every topos is well-pointed internally). This is equivalent to a certain formulation of type theory, so it is (in a sense) nothing new, although it leads to new perspectives, as in the next paragraph.

Categories (not just toposes) can serve as models of type theories, each type theory corresponding to a certain class of categories. Toposes correspond directly to a constructive but impredicative type theory; to make the theory predicative (in the constructivists' sense) you generalise to a pretopos (maybe locally cartesian closed), to make the theory nonconstructive you specialise to a Boolean topos, and so on. More specifically, every category's internal language is a type theory (with many odd constants), and every type theory (of appropriate form) defines a category (its free model); this is an adjunction between categories and type theories. Paul Taylor's book Practical Foundations of Mathematics is essentially all about this subject, as is (at a more advanced level) most of the career of Michael Makkai.

Jim Lambek proposed to use the free topos as ambient world to do mathematics in. Being syntactically constructed, but universally determined, with higher-order intuitionistic type theory as internal language he saw it as a reconciliation of the three classical schools of philosophy of mathematics, namely formalism, platonism, and intuitionism. His latest views on this variant of categorical foundations can be found in (Lambek-Scott 2011).

Certain ‘strong’ axioms of set theory (those involving quantification over all sets) are difficult to state in category-theoretic (or type-theoretic) terms, but this can be overcome in a theory like ETCS; talk to Mike Shulman. (Ironically, this makes it harder to do foundations with categorial foundations!)

In contrast, many of the optional or controversial axioms of set theory (such as the axiom of choice) can be stated quite directly in ETCS. These can be examined quite well in a naïve set-theoretic language that never need be precise about whether one's foundations are traditional (membership-based), categorial, or whatever.

## Categorial foundations of category theory

Relevant topics: ETCC, FOLDS, homotopy type theory

### General

• William Lawvere, Foundations and applications: axiomatization and education, Bulletin of Symbolic Logic 9 (2003), 213-224 (ps, Euclid)

The following contains a careful discussion of Gödel's incompleteness theorem in the context of categorical foundations using the free topos:

• Jim Lambek, Phil Scott, Reflections on Categorical Foundations of Mathematics , pp.171-185 in Sommaruga (ed.), Foundational Theories of Classical and Constructive Mathematics, Springer New York 2011. (draft)

Some old discussions about category theory and foundations are archived here