If your philosophical edifice rests on the idea of the Good, and you believe that wisdom consists in knowing and doing the Good, then it is incumbent upon you at some point to explain what it means to know the Good. In order to do that, you probably need to explain what it means to know anything at all. You need to answer the question, "what is knowledge?". Plato asks that question in the Theaetetus. If answering it requires giving a (...) definition of knowledge in general, he fails to give an answer. Worse, the dialogue suggests that it is impossible to define knowledge. The result is a failure not only for epistemology, but for any politics or ethics based on knowledge of the Forms. The pursuit of wisdom falls by the wayside, unmasked as quixotry. ;I argue that there are several long and important passages in the Theaetetus which are centrally concerned with the question of how the pursuit of wisdom can be rescued from the failure of epistemology. To that end, I focus on the definition of knowledge as perception, and argue that Plato means its failure to reveal problems which will plague any attempt to define knowledge. I then analyse two passages which make wisdom their central concern, namely the "Apology of Protagoras" and the "Digression" . Finally, I suggest that Plato provides a model for understanding the rationally inaccessible nature of knowledge and its relation to wisdom. The model is given in Theaetetus' definition of irrational square roots, which is offered at the outset of the discussion as a paradigm for defining knowledge. Theaetetus defines irrational roots in terms of their productive powers; this provides a model for wisdom, insofar as wisdom is a kind of knowledge which produces practical activity, i.e., insofar as to know the Good is to do the Good. If the knowledge contained in wisdom cannot be defined in itself, it can be defined through its productive power. (shrink)