Russell's paradox
This axiom schema was tacitly used in the early days of naive set theory, before a strict axiomatization was adopted. Unfortunately, it leads directly to Russell's paradox by taking "φ"("x") to be ¬("x" ∈ "x") (i.e., the property that set "x" is not a member of itself). Therefore, no useful axiomatization of set theory can use unrestricted comprehension. Passing from classical to intuitionistic logic does not help, as the proof of Russell's paradox is intuitionistically valid.