Zermelo–Fraenkel set theory

3. Axiom schema of specification (also called the axiom schema of separation or of restricted comprehension)

Note that the axiom schema of specification can only construct subsets, and does not allow the construction of entities of the more general form:

ZFC has been criticized both for being excessively strong and for being excessively weak, as well as for its failure to capture objects such as proper classes and the universal set.