In "Philosophy" (1929) Ramsey states that "Logic issues in tautologies, mathematics in identities, philosophy in definitions; all trivial but all part of the vital work of clarifying and organizing our thought. " I'm assuming that Ramsey means to identify the central product in each of these fields, otherwise the statement reads like a platitude -- sure, Ramsey was just twenty-five when he made the comment, but we're talking about someone who D.H. Mellor, in his introduction to Philosophical Papers, seems to place above the likes of Moore, Russell, Whitehead, and Wittgenstein. In any case, if Ramsey intended something along the lines of the former, then his statement strikes me as wrong, at least from a modern view. Sometimes an important mathematical contribution comes in the form of a definition, as the successful isolation of a powerful idea. For example, consider some of the basic definitions from computability theory or, perhaps closer to mathematics proper, some of the fundamental ideas from category theory (e.g. natural transformation, adjoint functor).