Axiom:Axiom of Specification/Set Theory

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The axiom of specification is an axiom schema which can be formally stated as follows:

For any function of propositional logic $\map P y$, we introduce the axiom:

$\forall z: \exists x: \forall y: \paren {y \in x \iff \paren {y \in z \land \map P y} }$

where each of $x$, $y$ and $z$ range over arbitrary sets.

This means that if you have a set, you can create a set that contains some of the elements of that set, where those elements are specified by stipulating that they satisfy some (arbitrary) condition.

Also presented as

The axiom of specification can also be specified as follows:

If $\phi$ is a property (with parameter $p$), then for any $X$ and $p$ there exists a set:
$Y = \paren {u \in X: \map \phi {u, p} }$
that contains all those $u \in X$ that have the property $\phi$.
$\forall X: \forall p: \exists Y: \forall u: \paren {u \in Y \iff \paren {u \in X \land \map \phi {u, p} } }$

Also known as

The axiom of specification is also known as:

  • In the context of class theory, the term axiom of class formation is often seen.

Also see

Historical Note

The axiom of specification was created by Ernst Zermelo as a replacement for the comprehension principle of Frege set theory.

The latter had been demonstrated, via Russell's Paradox, to lead to the conclusion that Frege Set Theory is Logically Inconsistent.

Thus, rather than allowing a set to be constructed of any elements at all which satisfy a given property $P$, the elements in question are restricted to being elements of some pre-existing set.

This in turn leads to the further question of how to create such a pre-existing set in the first place.

Hence the need to develop further axioms in order to allow the creation of such sets.

As a result of this, Ernst Zermelo found it necessary to create:

the axiom of existence, allowing for the existence of $\O := \set {}$
the axiom of pairing, allowing for $\set {a, b}$ given the existence of $a$ and $b$
the axiom of unions, allowing for $\bigcup a$ given the existence of a set $a$ of sets
the axiom of powers, allowing for the power set $\powerset a$ to be generated for any set $a$
the axiom of infinity, allowing for the creation of the set of natural numbers $\N$.


Axiom of specification is translated:

In German: Aussonderungsaxiom  (literally: axiom of segregation)