According to the sophist Protagoras, reality seems stable to us only because our linguistic expressions and forms are constant. In itself, however, reality is subject to constant changes. Thus, everything we say would be false in the sense that it has nothing to do with reality. Metaphysics, as practiced by Plato and Aristotle, is the theoretical endeavor that aims to refute this sophistic view of language and reality. It therefore seeks to conceive enduring structures of all that is real through reflection on certain forms of thinking and speaking, arguing that nothing can exist that is not so structured. The task of metaphysical theories is thus to establish the possibility of true statements and, beyond that, of knowledge. This book introduces the philosophical discipline.