Gettier objects the definition of knowledge as justified true belief, and mentions Plato's Theaetetus 201 as an ancient source of such definition. In fact, Plato objects some such definition in the dialogue, too. This paper will firstly explore Plato's arguments against such definition. It will secondly explore the definition discussed in the dialogue. This will show that Plato argues in quite a different way from modern epistemological discussions. The further question is what Platonic knowledge actually is. Since the dialogue ends without any firm conclusion, scholars have difficulties to define Platonic knowledge. However, this is consistent with Plato's other views on knowledge. If the dialogue provides any definition, it may merely persuade us with some belief, but not make us know. The aporetic ending of Theaetetus is an important key. Plato must show the way to search knowledge without offering a persuasive answer. From such a view, the way in which Plato searches the definition of knowledge is more significant rather than a definition of Platonic knowledge.