Well, in predicate logic you have individuals that have/satisfy a property/predicate. I propose that the property is the set of these individuals. — litewave
Cool. Let's look at this.
If the property is "is the set of these individuals", effectively F in F(x), what is the individual which satisfies this predicate?
It's interesting to try and think of sets in terms of predicate logic -- and I can see the analogy between a predicate and a set since we can quantify over both and make valid deductions between those quantifications. So the temptation is strong to equate a set with a predicate.
The way I was taught*, at least, sets are different from propositions are different from predicates, but they can have relations to one another. If I were to render set theory in terms of predicates I would say "Set theory is the study of validity of the "is a member of" relation", whereas predicate logic is the study** of validity between predicates. So they're kind of just asking after different things -- one is "how do we draw valid inferences between two propositions?" and the other is "how do we draw valid inferences between collections of individuals?"
Now, interestingly, I think we
can mix these logics sometimes -- but
usually we want to keep them distinct because they're hard enough to understand as it is that it's better to not overgeneralize
:D
I'd argue what this shows is that logic is something we choose to utilize. I'm not sure there really is some universally true thing we can say about sets and predicates sans the rest of the logical system. We could choose, for whatever reason (perhaps because this question is interesting and we're interested in how logic works), to
start with the equation "Properties are equivalent to sets" and then work out the validity of
that identification.
But, at least if we're learning, these are generally treated somewhat separately (even though, yes, there's a lot of overlap between these ways of talking at the intuitive level)
**EDIT: OK,
is the result of the study.... a theory is not an -ology
*EDIT: Also, "taught" was by a math instructor and the rest is self-study, so I could be missing something. I don't want to lead people astray but I do like thinking about this stuff.