Theaetetus' first definition : logos ou phaulos
Socrates and Theaetetus consider and reject three different definitions of knowledge in the Theaetetus. The first of these is the thesis that knowledge is perception. According to the received reading Plato's consideration of the thesis that knowledge is perception is limited to the consideration of the naive and implausible thesis that immediate sense-perception is knowledge and there is no knowledge apart from immediate sense-perception. This reading, which limits the philosophic interest of Platos consideration of the thesis that knowledge is perception, follows from a widespread misunderstanding of Socrates' reasons for introducing Protagoras and Heraclitus which circumscribes their role in the dialogue to supplying two theses, epistemological relativism and metaphysical flux, which are sufficient or perhaps necessary conditions for the thesis that knowledge is perception. I will show that Socrates introduces Protagoras and Heraclitus, not simply because they provide the epistemological or metaphysical framework within which Theaetetus' definition holds good, but because each man is committed to the thesis that knowledge is perception. Protagoras' sophistic expertise will be classed as a kind of empirical knowledge which bases itself on past and present perceptions and makes educated predictions of future perceptions. While Heraclitus' theory of flux will lead to a radical skepticism which rejects the possibility that there should be any knowledge of the world apart from perception. Socrates will give arguments against both of these ways of understanding the thesis that knowledge is perception. Plato thus articulates, develops, and ultimately rejects three different ways of understanding Theaetetus' initial definition of knowledge.