In this paper, I examine several key issues relating to the definition of knowledge as perception in the first part of Plato's Theaetetus. I begin by explaining the workings of the ‘secret doctrine’ of perception, which is introduced in order to support the idea that perception is incorrigible, and then turn to examine the two refutations of the definition of knowledge as perception which appear at the end of the first part of the Theaetetus. I shall present and explain distinct (...) lines of interpretation pertaining to these passages and explore the consequences they attempt to draw regarding Plato's views on the perceptual world, the nature of perception and how these bear on his conception of knowledge. (shrink)