# nLab axiom of extensionality

The axiom of extensionality

# The axiom of extensionality

## Idea

In material set theory, the axiom of extensionality says that the global membership relation $\in$ is an extensional relation on the class of all pure sets.

Since any relation becomes extensional on its extensional quotient, one can interpret this axiom as a definition of equality. However, because the extensional quotient map need not reflect the relation, there is still content to the axiom: if two sets would be identified in the extensional quotient, then they must be members of the same sets and have the same sets as members.

If one models pure sets in structural set theory, then this property may be made to hold by construction.

## Statements

### Weak extensionality

#### In unsorted set theories

In any unsorted set theory, the axiom of weak extensionality states that given a set $A$ and a set $B$, $A = B$ if and only if for all $C$, $C \in A$ if and only if $C \in B$.

If the set theory does not have equality as a primitive, we could define equality as the predicate

$A = B \coloneqq \forall C.(C \in A) \iff (C \in B)$

The axiom of weak extensionality is a foundational axiom in most material set theories, such as Zermelo set theory and Mac Lane set theory, both which do not have the axiom of foundation, as well as ZFC which does have the axiom of foundation.

In fully formal ETCS, where the basic objects of the theory are functions, $0$ represents both the empty set and the identity function on the empty set, and $1$ represents the singleton, the identity function of the singleton, and the sole element of the singleton all at the same time, there are three possible notions of sets:

• as identity functions $\mathrm{set}(a) \coloneqq \mathrm{dom}(a) = a$ or $\mathrm{set}(a) \coloneqq \mathrm{codom}(a) = a$

• as functions into the singleton $\mathrm{set}(a) \coloneqq (\mathrm{codom}(a) = 1)$

• as functions from the empty set $\mathrm{set}(a) \coloneqq (\mathrm{dom}(a) = 0)$

The elements are functions with domain $1$, $\mathrm{element}(a) \coloneqq (\mathrm{dom}(a) = 1)$

When sets are defined as functions into the singleton, the membership relation $a \in b$ is defined by the function $a$ being an element, the function $b$ being a set, and the codomain of $a$ being $b$

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

Weak extensionality is a theorem in this case. By the universal property of the singleton, any two sets $A$ and $B$ with the same domain are equal to each other, which means that any proposition $P$ between $A$ and $B$ implies that $A = B$, and thus that $\forall C.(C \in A) \iff (C \in B)$ implies that $A = B$. The converse follows from indiscernibility of identicals.

#### In two-sorted set theories

In any two-sorted set theory, the axiom of weak extensionality states that given a set $A:Set$ and a set $B:Set$, $A =_{Set} B$ if and only if for all $a:Element$, $a \in A$ if and only if $a \in B$.

If the set theory does not have equality of sets as a primitive, we could define equality of sets as the predicate

$A =_{Set} B \coloneqq \forall a:Element.(a \in A) \iff (a \in B)$

### Material strong extensionality

Let $\sim$ be a bisimulation, a binary relation such that for all sets $A$ and $B$ such that $A \sim B$, the following conditions hold:

• for all sets $C$ such that $C \in A$, there exists a set $D$ such that $D \in B$ and $C \sim D$
• for all sets $D$ such that $D \in B$, there exists a set $C$ such that $C \in A$ and $C \sim D$

The axiom of strong extensionality states that for every bisimulation $\sim$ and for every set $A$ and $B$, $A \sim B$ implies that $A = B$.

If the set theory does not have equality as a primitive, we could define equality as the terminal bisimulation, as the bisimulation $=$ such that for every bisimulation $\sim$ and for every set $A$ and $B$, $A \sim B$ implies that $A = B$.

In any set theory with the axiom of foundation, the axiom of weak extensionality implies the axiom of strong extensionality.

### Structural strong extensionality

In any structural set theory, the axiom of strong extensionality states that for all sets $A$ and $B$ with an injection $i:A \hookrightarrow B$, the two definitions of $i$ being a bijection are logically equivalent to each other:

• there exists a function $i^{-1}:B \to A$ such that for all elements $a \in A$ and $b \in B$, $i^{-1}(i(a)) = a$ and $i(i^{-1}(b)) = b$
• for every element $x \in B$ there exists an element $y \in A$ such that $i(y) = x$.

Similar to how in material set theory one can use the axiom of extensionality to define equality of sets, in structural set theory one can use the axiom of strong extensionality to define bijection of sets.