In this book I have sketched the principles of the mental life of groups and have made a rough attempt to apply these principles to the understanding of the life of nations. I have had the substance of the book in the form of lecture notes for some years, but have long hesitated to publish it. I have been held back, partly by my sense of the magnitude and difficulty of the subject and the inadequacy of my own preparation for dealing with it, partly because I wished to build upon a firm foundation of generally accepted principles of human nature. Some fifteen years ago I projected a complete treatise on Social Psychology which would have comprised the substance of the present volume. I was prevented from carrying out the ambitious scheme, partly by the difficulty of finding a publisher, partly by my increasing sense of the lack of any generally accepted or acceptable account of the constitution of human nature. I found it necessary to attempt to provide such a foundation, and in 1908 published my Introduction to Social Psychology. That book has enjoyed a certain popular success. But it was more novel, more revolutionary, than I had supposed when writing it; and my hope that it would rapidly be accepted by my colleagues as in the main a true account of the fundamentals of human nature has not been realised. All this part of psychology labours under the great difficulty that the worker in it cannot, like other men of science, publish his conclusions as discoveries which will necessarily be accepted by any persons competent to judge. He can only state his conclusions and his reasonings and hope that they may gradually gain the general approval of his colleagues. For to the obscure questions of fact with which he deals it is in the nature of things impossible to return answers supported by indisputable experimental proofs. In this field the evidence of an author's approximation towards truth can consist only in his success in gradually persuading competent opinion of the value of his views. My sketch of the fundamentals of human nature can hardly claim even that degree of success which would be constituted by an active criticism and discussion of it in competent quarters. Yet there are not wanting indications that opinion is turning slowly towards the acceptance of some such doctrine as I then outlined. Especially the development of psycho-pathology, stimulated so greatly by the esoteric dogmas of the Freudian school, points in this direction. The only test and verification to which any scheme of human nature can be submitted is the application of it to practice in the elucidation of the concrete phenomena of human life and in the control and direction of conduct, especially in the two great fields of medicine and education. And I have been much encouraged by finding that some workers in both of these fields have found my scheme of use in their practice and have even, in some few cases, given it a cordial general approval. But group psychology is itself one of the fields in which such testing and verification must be sought. And I have decided to delay no longer in attempting to bring my scheme to this test. I am also impelled to venture on what may appear to be premature publication by the fact that five of the best years of my life have been wholly given up to military service and the practical problems of psycho-therapy, and by the reflection that the years of a man's life are numbered and that, even though I should delay yet another fifteen years, I might find that I had made but little progress towards securing the firm foundation I desired.