This book explains how the idea of there being no ultimate, universal truth is in itself a contradictory philosophical position. Philosophers throughout history have tried to answer the question of what the deepest truth is about the world and our situation in it. They have put forward various philosophical systems, but none have won universal acceptance; and it is widely believed today that the time of philosophical systems is over. This book shows very concretely how to proceed in order to uncover the system of mutually defined basic concepts, which must be presupposed for any possible description of reality and any possible description of our situation as persons. Thus, in reigniting the discussion on philosophical systems, the author adds to the historical and contemporary discussions of Kant's structure of pure reason.