The Law of Non-Contradiction has been high orthodoxy in Western philosophy since Aristotle. The so-called Law has been the subject of radical challenge in recent years by dialetheism, the view that some contradictions are indeed true. Many philosophers have taken the Law to be central to many of our most important philosophical concepts. In Doubt Truth to be a Liar, Graham Priest mounts the case against this. Starting with an analysis of Aristotle on the Law, he discusses the nature of truth, or rationality, or negation, and of logic itself, and argues that the Law is inessential to all of these things. The book takes off from Priest's earlier book, In Contradiction (a second edition of which is also published by OUP), developing its themes largely without recourse to formal logic.The book is required reading for anyone who wishes to understand dialetheism; (especially) for anyone who wishes to continue to endorse the old Aristotelian orthodoxy; and more generally, for anyone who wishes to understand the role that contradiction plays in our thinking.