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. It may seem to some minds astonishing that I should now admit that the substance of this book was committed to writing before the Great War; for that war is supposed by some to have revolutionised all our ideas of human nature and of national life. But the war has given me little reason to add to or to change what I had written. This may be either because I am too old to learn, or because what I had written was in the main true; and I am naturally disposed to accept the second explanation.