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Trouble comprehending the empty set

  1. May 28, 2008 #1
    What exactly is a set with no elements? What does it mean? Aren't sets entirely defined by their contents? In what manner is a set with no contents defined?

    I'm not arguing for or against such an axiom... I simply want to know what the axiom actually is saying. If an empty set is not well-defined, what is it actually asserting?
     
  2. jcsd
  3. May 29, 2008 #2
    The empty set contains the elements that are the intersection of two mutually exclusive events.
     
  4. May 29, 2008 #3

    Hurkyl

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    Just what you said. If A is an empty set, then, for any x, x is not in A.

    What do you mean by 'mean'? i.e. what sort of semantic interpretation are you using?

    Two sets with the same membership relation are the same set, if that's what you mean.

    For example, by the condition that for any x, x is not a member of that set.

    It's asserting that an empty set exists.
     
  5. May 29, 2008 #4

    CRGreathouse

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    The empty set is the set of McLaren F1s I own. The empty set is the intersection of the set of red things and the set of nonred things.

    Sets are defined on the basis of their contents, which means that:
    * The empty set makes sense (it is the set which has no elements)
    * The empty set is unique (if there were two empty sets, they would contain the same things [nothing] and so be equal)
     
  6. May 29, 2008 #5
    A set is indeed defined entirely by its elements. Which is why the empty set is unique.
     
  7. May 29, 2008 #6
    I prefer to say that the empty set is unique... up to equality.

    After all, there is nothing in set theory to say that there is one and only one of each set. It's just that if there were multiple copies of a set, nothing in set theory could at all tell them apart.

    I could potentially have a model of set theory where there are 2 empty sets but they are treated identically. Then most sets would have many equal copies. And I could only tell them apart if I had some way outside of set theory that could distinguish them.
     
  8. May 29, 2008 #7

    CRGreathouse

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    Ontologically, it's usual to treat equality as more than just an equivalence relation, but certainly nothing bad would happen if you used an equivalence relation over sets where things that are equivalent 'act the same way' set-theoretically.
     
  9. May 30, 2008 #8
    Running on empty?

    "What exactly is a set with no elements? What does it mean? Aren't sets entirely defined by their contents? In what manner is a set with no contents defined?"

    A more tangible speculative physical model might be to take a universe and toss away quanta, and patches of manifold. Supposedly nothing left, it might seem.
     
  10. Jun 2, 2008 #9
    Thanks, everyone, for your replies.

    However, I'm still having trouble. This is really hard for me, and I feel dumb for failing to perceive something so simple. Here's my next question that may put the issue to rest, for me.

    What would be a good definition for empty set, that relies in no way on a notion of a non-empty set, or a definition of a non-empty set, that relies in no way on a notion of an empty set? Surely, one of these must hold or the definitions are circular.
     
  11. Jun 3, 2008 #10

    CRGreathouse

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    The empty set is the set that contains exactly those elements that are not equal to themselves.
     
  12. Jun 3, 2008 #11

    HallsofIvy

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    ??? On the contrary, if neither definition relies on the other the definitions can't be circular. The definitions would only be circular if each depended on the definition of the other!
     
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