Talk:Axiom schema of specification
|WikiProject Mathematics||(Rated B-class, Low-importance)|
Unrestricted comprehension and paradoxes 
Concerning "unrestricted comprehension", PlanetMath's "comprehension axiom" says:
- In theories which make no distinction between objects and sets (such as ZF), this formulation leads to Russel's paradox, however in stratified theories this is not a problem (for example second order arithmetic includes the axiom of comprehension).
- This axiom schema was tacitly used in the early days of naive set theory, before a strict axiomatisation was adopted. Unfortunately, it leads directly to Russell's paradox by taking P(C) to be (C is not in C). Therefore, no useful axiomatisation of set theory can use unrestricted comprehension, at least not with classical logic.
I don't see any contradiction here. NF's comprehension axiom is not unrestricted;it's limited to stratified formulas. It's true that it uses urestricted quantification, but not what I'd call unrestricted comprehension. --Trovatore 21:27, 4 October 2005 (UTC)
- Oh, you weren't talking about NF. Hmm. Well, second-order arithmetic isn't a (general) set theory, exactly. I'm really not all that familiar with the system of Principia. --Trovatore 21:30, 4 October 2005 (UTC)
The version on the ZFC page is different than the one on this page, and this page doesn't even list the version they have. I changed ZFC's to reflect this page's version, but it was quickly reverted. So I'm wondering if this page's version should instead be changed to the one ZFC has?
- Nothing wrong with either version. All the ones on the ZFC page should probably pick one version consistently, but that doesn't mean this page should show only that version. Maybe, like axiom of choice, we could show many different versions.