 One early skeptic concerning an unrestricted Comprehension (or Abstraction) axiom was the originator of modern set theory, Georg Cantor. Even prior to Russell’s discovery, Cantor had rejected unrestricted Comprehension in favour of what was, in effect, a distinction between sets and classes, recognizing that some properties (such as the property of being an ordinal) produced collections that were simply too large to be sets, and that any assumption to the contrary would lead to inconsistency. (Details can be found in Moore (1982), Hallett (1984) and Menzel (1984).) 