"What is knowledge?" Plato does try to answer this question, asked at the beginning of the Theaetetus, but the answer is not in the dialogue itself, either negatively (as Cornford argued) or positively (as Fine suggested). His answer is partially given in the Sophist and Statesman: the project of definition has been shown to involve the mastery of the whole field to which the object of definition belongs, and hence a science of the field in question. The dramatic sequels to the Theaetetus are also its doctrinal complements. By making knowledge the object of knowledge, Plato was able to exhibit both the correct method and the content of dialectic, which he took as the very essence of knowledge itself.
Nehamas, Alexander, "Episteme and Logos in Plato's Later Thought" (1981). The Society for Ancient Greek Philosophy Newsletter. 121.