Rush Rhees, a close friend of Wittgenstein and a major interpreter of his work, shows how Wittgenstein's On Certainty concerns logic, language, and reality - topics that occupied Wittgenstein since early in his career. Authoritative interpretation of Wittgenstein's last great work, On Certainty, by one of his closest friends. Debunks misconceptions about Wittgenstein's On Certainty and shows that it is an essay on logic. Exposes the continuity in Wittgenstein's thought, and the radical character of his conclusions. Contains a substantial and illuminating afterword discussing current scholarship surrounding On Certainty, and its relationship to Rhees's work on this subject.