This paper tries to establish the following three points about Wittgenstein's On Certainty. The first is that this book is not a collection of fragments about knowledge but the record of Wittgenstein's systematic thinking that aims at a single question. The second is that it is the explication of the meaning of particular certainty Moore showed when he stated he knew, e.g., he had hands that Wittgenstein's thinking aimed at. The third is that Wittgenstein's answer to this question showed a dramatic shift from the "public" interpretation of Moore's certainty to the "private" one about 10 days before his death.
