Political problems, domestic or international, always involve us in psychological and philosophical issues; but they tend to develop whatever psychology or philosophy they need for their own purposes rather than to borrow from the formidable and not always luminous stock pile of professional literature in these subjects. This tendency to be one's own philosopher is entirely justified, for the simple reason that philosophy is always arising from experience, showing an inductive as well as a deductive side—and that the political experience of mankind is prolific in comment on the nature of man and of the world man lives in. Thomas Hobbes, for example, is a political writer whose contributions to psychology have not yet been fully appreciated.
At the same time, there are recent advances in both psychology and philosophy of which political science ought to enjoy the consequences. If any light is thrown on the nature of the human individual, that light is directly pertinent to political science. Enquiries into the nature of human association are similarly relevant.