The title of this chapter is that of Cornford’s translation of and commentary on the Theaetetus and the Sophistes. It is in the former dialogue that the ‘subjective’ side of the knower-known relation is discussed. Cornford’s view was that the Forms are deliberately excluded by Plato from the Theaetetus in order to show that sensation, opinion, and opinion-plus-definition cannot be knowledge, and therefore that only cognizance by intellect of the Forms can be knowledge. This view has not proved acceptable, though if the dialogue Philosopher had ever appeared (as promised in Sophistes and Politicus) we might well have seen that the Theaetetus did in a sense provide prolegomena to a positive theory of knowledge which entailed some statement also of a doctrine of Forms as objects of knowledge.