Naive set theory

Naive set theory is an informal approach to set theory that deals with the basic concepts and principles of sets without the rigorous formalism found in axiomatic set theory, such as Zermelo-Fraenkel set theory with the Axiom of Choice (ZFC). In naive set theory, a set is generally defined intuitively as a collection of distinct objects, which can be anything: numbers, symbols, points, or even other sets.