It is important for our purposes to notice that in this first reduction of Theætetus' definition of knowledge as perception, Plato has introduced the distinction between sense object and physical object, for he has specifically said, "when the same wind is blowing, one of us feels chilly, the other does not." In using this example. Plato has, as Cornford observes, raised the question of how the several sense objects are related to the single physical object. This question is one of (...) the two major questions with which this paper will deal, since it is, as I have remarked, the more important question concerning the ontological status of sense-data. Cornford feels that Protagoras probably held that the physical object is both hot and cold, different observers perceiving different aspects of the same object. That is, Cornford suggests that Protagoras' own answer to this question of the relation between sense-data and physical objects is one of complete identification in every case. We will see that this theory is similar in many ways to the one worked out by Plato in the Theætetus, but the distinction between the physical object and the sense object, between the wind which feels hot and cold to different people, makes certain characteristic differences between Plato's and Protagoras' theory. Before going on to examine these differences, it is necessary to follow the further development of Plato's general theory of perception. (shrink)

Cornford assures us that the theory of sensation developed in the Theaetetus must be Plato's own. He offers three considerations in support of this contention.