Axiom Schema of Specification - Statement

Statement

One instance of the schema is included for each formula φ in the language of set theory with free variables among x, w1, ..., wn, A. So B is not free in φ. In the formal language of set theory, the axiom schema is:

or in words:

Given any set A, there is a set B such that, given any set x, x is a member of B if and only if x is a member of A and φ holds for x.

Note that there is one axiom for every such predicate φ; thus, this is an axiom schema.

To understand this axiom schema, note that the set B must be a subset of A. Thus, what the axiom schema is really saying is that, given a set A and a predicate P, we can find a subset B of A whose members are precisely the members of A that satisfy P. By the axiom of extensionality this set is unique. We usually denote this set using set-builder notation as {C ∈ A : P(C)}. Thus the essence of the axiom is:

Every subclass of a set that is defined by a predicate is itself a set.

The axiom schema of specification is characteristic of systems of axiomatic set theory related to the usual set theory ZFC, but does not usually appear in radically different systems of alternative set theory. For example, New Foundations and positive set theory use different restrictions of the axiom of comprehension of naive set theory. The Alternative Set Theory of Vopenka makes a specific point of allowing proper subclasses of sets, called semisets. Even in systems related to ZFC, this scheme is sometimes restricted to formulas with bounded quantifiers, as in Kripke–Platek set theory with urelements.

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