|Classes: identity of classes provided by same elements (extension) - identity of properties by the same predicates (intension).|
Books on Amazon
Class / Prior: we think of it as a term for what we describe as identical if condition (2) is satisfied: (2) PxPyEfxyyxy - ((s) why two-place predicate?).
Objects of thought Oxford 1971
Arthur N. Prior
Papers on Time and Tense 2nd Edition Oxford 2003