Definitions can be classified from two different points of view. We can ask what sort of statements definitions are, how they are to be justified, and what purpose they serve in the process of acquiring scientific knowledge. For lack of a simpler word, let us call a classification of definitions from this point of view epistemological. We can also distinguish different forms of definition; and a classification from this point of view is naturally called formal.

The repudiation of propositions as “obscure entities” which is prevalent among logicians and philosophers of “nominalistic” persuasion, is frequently justified by pointing out that no agreement seems ever to have been reached about the identity-condition of propositions. And if we cannot specify, so they argue, under what conditions two sentences express the same proposition, then we use the word “proposition” without any clear meaning. Quine, for example, feels far less uneasy about quantification over class-variables than about quantification over attribute-variables and (...) propositional variables, because there is a clear criterion for deciding whether we are dealing with two classes or with only one class referred to by different predicates: two classes are identical if they have the same membership. And such a criterion of identity is alleged to be lacking for intensions. (shrink)