Contents

foundations

# Contents

## Idea

There are two general approaches to structural set theory; those that attempt to axiomatise the category Set of sets and functions, and those that attempt to axiomatise the allegory Rel of sets and relations. The former is called categorical set theory.

## Types of categorical set theories

One distinguishes between simply sorted categorical set theories, where membership is a relation, and dependently sorted categorical set theories, where membership is a typing judgment.

### Simply sorted categorical set theories

Simply sorted categorical set theories have a sort of possible sets and a sort of possible elements, and a membership relation between the sort of possible sets and the sort of possible elements. By possible set we mean that some of the terms in the sort might be a set; and by possible element we mean that some of the terms in the sort might be an element.

• In unsorted categorical set theory, there is only a sort or judgment of functions, and the sort/judgment of functions plays the role of both the sort of possible sets and the sort of possible elements.
• In two-sorted categorical set theory, there are sorts for functions and sets, the sort of sets plays the role of the sort of possible sets, and the sort of functions plays the role of the sort of possible elements.
• In three-sorted categorical set theory, there are sorts for functions, sets, and elements, and the sort of sets plays the role of the sort of possible sets, and the sort of elements plays the role of the sort of possible sets.

#### Unsorted categorical set theory

Unsorted categorical set theory only consists of functions as primitive judgments or sorts. The elements are defined as functions with singleton domain ($\mathrm{element}(a) \coloneqq (\mathrm{dom}(a) = 1)$) However, there are multiple ways of defining sets in unsorted categorical set theory, each which lead to a different categorical set theory; examples include but are not limited to

• identity functions ($\mathrm{set}(a) \coloneqq \mathrm{dom}(a) = a$ or $\mathrm{set}(a) \coloneqq \mathrm{codom}(a) = a$)
• functions with singleton codomain ($\mathrm{set}(a) \coloneqq \mathrm{codom}(a) = 1$)
• functions with empty set domain ($\mathrm{set}(a) \coloneqq \mathrm{dom}(a) = 0$)

In any unsorted first-order categorical set theory, the membership relation is a defined global relation on the functions, rather than a primitive global relation as in material set theory. For each definition of set, there is a definition of membership relation:

• for sets as identity functions, given functions $a$ and $b$, the membership relation is defined as $a$ being an element, $b$ being a set, and the codomain of $a$ being equal to $b$:

$a \in b \coloneqq \mathrm{element}(a) \wedge \mathrm{set}(b) \wedge \mathrm{codom}(a) = b$
• for sets as functions into the singleton, given functions $a$ and $b$, the membership relation is defined as $a$ being an element, $b$ being a set, and the codomain of $a$ being equal to the domain of $b$:

$a \in b \coloneqq \mathrm{element}(a) \wedge \mathrm{set}(b) \wedge \mathrm{codom}(a) = \mathrm{dom}(b)$
• for sets as functions out of the empty set, given functions $a$ and $b$, the membership relation is defined as $a$ being an element, $b$ being a set, and the codomain of $a$ being equal to the codomain of $b$:

$a \in b \coloneqq \mathrm{element}(a) \wedge \mathrm{set}(b) \wedge \mathrm{codom}(a) = \mathrm{codom}(b)$

While that membership relation can be proven to satisfy the axiom of weak extensionality, it cannot in general be proven to satisfy the axiom of strong extensionality. In addition, depending on the definition of the set, the membership relation has different properties:

• for sets as identity functions, every set is a Quine atom with respect to $\in$
• for sets as functions with singleton codomain, only sets in bijection with the singleton is a Quine atom with respect to $\in$
• for sets as functions with empty set domain, no set is a Quine atom with respect to $\in$

The canonical example of an unsorted first-order categorical set theory is fully formal ETCS. An axiom schema of collection or axiom schema of replacement could be added to fully formal ETCS to get an unsorted categorical set theory which is equivalent in strength to ZFC.

#### Two-sorted categorical set theory

Two-sorted categorical set theory consists of sets and functions as primitive judgments or sorts. The elements are defined as functions with singleton domain.

The canonical example of this is the two-sorted first-order theory ETCS of sets, functions, identity functions, and composition of functions. An axiom schema of collection or axiom schema of replacement could be added to ETCS to get an two-sorted categorical set theory which is equivalent in strength to ZFC.

#### Three-sorted categorical set theory

Three-sorted categorical set theory consists of sets, functions, and elements as primitive judgments or sorts. An example of a three-sorted categorical set theory is the simply-sorted presentation of structural ZFC.

### Dependently sorted categorical set theory

Two-sorted categorical set theory can also be expressed as a dependently sorted categorical set theory, where there are sets, and there are functions which depend on a pair of sets.

An example of this is the dependently sorted version of ETCS.

Three-sorted categorical set theory can also be expressed as a dependently sorted categorical set theory, where there are sets, there are elements which depend on a set, and there are functions which depend on a pair of sets.

An example of this is ETCS with elements, which defines the theory of a concrete well-pointed topos with natural numbers object and the axiom of choice.

An axiom schema of collection or axiom schema of replacement could be added to ETCS with elements to get an two-sorted categorical set theory which is equivalent in strength to ZFC.