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I am learning the axioms of Zermelo-Fraenkel (ZF) set theory.

One axiom schema basically says that given any set S and any formula phi(x), there is a set T consisting of all those elements x of S such that phi(x).

I find this axiom schema unsatisfying because it only guarantees that subsets of S definable by a formula are really sets. It's like saying that a function from X to Y only exists if you can write down a formula for it rather than just allowing arbitrary single-valued subsets of X cross Y. Is there a way in the language of ZF to say "given a set S, if T is a subset of S then T is a set?"

Related is the power set axiom. Given a set S, there is a set P(S) consisting of exactly all the subsets of S. But in ZF the objects of the theory are sets (no urelements). Everything is a set. So, the elements of P(S) are all sets. Doesn't this mean that every subset of a given set S is a set? If so, why the need for the subset axioms?

I can anticipate some difficulty. I want to say that "if x is a member of P(S) then x is a set," but I cannot express the predicate "is a set" in the language of set theory, and strangely enough there is no need to since everything is a set! I attempted:

"(for all S)(for all x)[x subset S --> (there exists y)(x = y)]"

where "x subset S" is an abbreviation for "(for all z)[z in x --> z in S]."

But this attempt is silly because when I say "for all x," x is automatically a set and there is no need to say it is a set.

I'm confused.

  1. Are the subset axioms necessary?
  2. Is there a way to remove reference to a defining formula so that every subset of a given set is a set?
  3. If the answer to 1 is "yes" and the answer to 2 is "no" then why is set theory so weird as to allow only definable subsets on the one hand and yet allow for a set of all subsets on the other?
  • 3
    "Subset of" is an abbreviation for "is a set and each of its elements is an element of". So the Subset Schema is really about what subcollections are sets; "if T is a subset then it is a set" is a tautology. The point of the Power Set axiom is not that the elements of P(S) are sets, but that there is a set whose elements are exactly the subsets of S.2010-08-13
  • 1
    In response to 3. The subset axiom schema says we can make sets by finding all elements of another set that satisfy a predicate. It doesn't "allow only" these subsets because it doesn't say that there aren't other types of subset as well.2010-08-13
  • 0
    The subset axioms _do_ require the subsets to be definable by a formula. I know it is not an "if and only if" statement. I know that there may be other subsets.2010-08-15

3 Answers 3