The store of scientific knowledge is a general treasure-house from which all men may draw. And yet – perhaps because of the wide spread of scientific ideas – we seldom remind ourselves that the development of the stock of scientific ideas, the heritage of all men, has always been the work of a very small band. Men capable of great scientific effort are rare and, for their effectual working, an intellectual environment is needed that is well nigh as rare as themselves. Surely the contemplation of the conditions under which such men have laboured and lived, the examination of their training and mental history, of their ways of life, and of the human setting must be of some value to those who would follow in their footsteps or prepare others to do so. Nor is the reverse side of the picture without its lesson. The study of those social, economic and philosophical conditions that fail to produce effective scientific fruits, or that yield only bizarre or deformed products, can at least explain for us certain phases in the mental history of mankind. Thus the study of the scientific mood in its historical development needs little justification.