>>13905239>How are you supposed to give a formal definition of sets?Something is a set if a set theory calls it a set.
A theory T is a set theory if it has a relation < and a type Set such that:
1. For any two terms A and B which have type Set, if for every term x, x < A if and only if x < B, then A = B.
2. For any term A of type Set and any term z which can be on the left side of the relation <, there exists a term B of type Set such that for every term x, x < B if and only if either x < A or x = z.
3. There exists a term O of type Set such that for every term x, x < O is false.
