The evolution of Kraepelin's nosological principles.

Emil Kraepelin developed a new psychiatric nosology in the eight editions of his textbook. Previous papers have explored his construction of particular diagnoses, including dementia praecox and manic-depressive insanity. Here we are providing a close reading of his introductory textbook chapter, that presents his general principles of nosology. We identify three phases: 1) editions 1-4, in which he describes nosological principles in search of data; 2) editions 5-7, in which he declares the mature version of his nosological principles and develops new disease categories; 3) edition 8, in which he qualifies his nosological claims and allows for greater differentiation of psychiatric disorders. We propose that Kraepelin's nosology is grounded in three principles. First, psychiatry, like other sciences, deals with natural phenomena. Second, mental states cannot be reduced to neural states, but science will progress and will, ultimately, reveal how nature creates abnormal mental states and behavior. Third, there is a hierarchy of validators of psychiatric diagnoses, with the careful study of clinical features (signs, symptoms and course) being more important than neuropathologic and etiological studies. These three principles emerged over the course of the eight editions of Kraepelin's textbook and were informed by his own research and by available scientific methods. His scientific views are still relevant today: they have generated and, at the same time, constrained our current psychiatric nosology.