Medical science, whose range is as vast as its history is old, has led to the formation of a specific style in the grasping of its problems and of a specific way of treating medical phenomena, i.e. to a specific type of thinking. In substance such separateness of the way of thinking is nothing extraordinary. One has only to realize the difference between the way of thinking of a scientist and that of a humanist, even if the subject in question is the same: for example, how great is the difference, and how great is the impossibility of a direct juxtaposition, between psychology as science and as a branch of philosophy. Even the very subject of medical cognition differs in principle from that a scientific cognition. A scientist looks for typical, normal phenomena, while a medical man studies precisely the atypical, abnormal, morbid phenomena. And it is evident that he finds on this road a great wealth and range of individuality of these phenomena which form a great number, without distinctly delimited units, and abounding in transitional, boundary states. There exists no strict boundary between what is healthy and what is diseased, and one never finds exactly the same clinical picture again. But this extremely rich wealth of forever different variants is to be surmounted mentally, for such is the cognitive task of medicine. How does one find a law for irregular phenomena? — this is the fundamental problem of medical thinking. In what way should they be grasped and what relations should be adopted between them in order to obtain a rational understanding?