 Axiom schema of specification

For each formula $\phi$, the axiom schema of specification forms the subsetof any set $x$. When interpreting the schema inside a class model, the formula must be evaluated internally.

The Axiom Schema of Specification (also known as the Axiom Schema of Separation) is a fundamental principle in set theory, particularly in the context of Zermelo-Fraenkel set theory (ZF). It is one of the axioms that govern how sets can be constructed and manipulated within this framework. In essence, the Axiom Schema of Specification allows for the creation of a new set by specifying a property that its elements must satisfy.