 Some philosophers, notably W.V.O. Quine, recognize the existence of certain sorts of abstract entities but not others at least partly on the basis of concerns about identity conditions.  Quine granted the existence of sets, in part because they obey the extensionality axiom: sets are identical iff they have the same members.  When it came to properties, relations and propositions, however, he found no such clear criterion of identity.  The property of being a creature with a heart, he noted, is distinct from the property of being a creature with a kidney, even if all the same things exemplify the two properties. 