There is hardly another principle in physics with more far-reaching consequences than Pauli's exclusion principle. This book explores the principle's origin in the 1920s, its embedding into quantum mechanics, and later experimental validation. Using this crucial historic episode to reconsider Kuhn's view on scientific revolutions, the author defends the prospective rationality of this transition, focusing on how Pauli's principle emerged as a phenomenological rule 'deduced' from anomalous phenomena. The process of validation is reconstructed and analysed within the framework of 'dynamic Kantianism'. The book will interest philosophers, historians and physicists.