This book is an attempt to provide a philosophical answer to the simple question, "What is the law?" as well as address the various debates this question has spawned. Along the way, it develops a unique position within analytic jurisprudence by carefully distinguishing between a theory of the nature of a legal system and a theory of the nature of legal content (that is, of individual laws). Finally, it applies the framework established in the first part of the book to two substantive areas within legal theory: legal reasoning and international legal systems. The result is a unique introduction to the philosophy of law, one that presents and tests a theory of analytic jurisprudence, while it introduces students and others to this sub-field of philosophy.