In this little book I have attempted to deal with a difficult branch of psychology in a way that shall make it intelligible and interesting to any cultivated reader, and that shall imply no previous familiarity with psychological treatises on his part: for I hope that the book may be of service to students of all the social sciences, by providing them with the minimum of psychological doctrine that is an indispensable part of the equipment for work in any of these sciences. I have not thought it necessary to enter into a discussion of the exact scope of social psychology and of its delimitation from sociology or the special social sciences: for I believe that such questions may be left to solve themselves in the course of time with the advance of the various branches of science concerned. I would only say that I believe social psychology to offer for research a vast and fertile field, which has been but little worked hitherto, and that in this book I have attempted to deal only with its most fundamental problems, those the solution of which is a presupposition of all profitable work in the various branches of the science. If I have severely criticised some of the views from which I dissent, and have connected these views with the names of writers who have maintained them, it is because I believe such criticism to be a great aid to clearness of exposition and also to be much needed in the present state of psychology: the names thus made use of were chosen because the bearers of them are authors well known for their valuable contributions to mental science. I hope that this brief acknowledgment may serve as an apology to any of them under whose eyes my criticisms may fall. I owe also some apology to my fellow-workers for the somewhat dogmatic tone I have adopted. I would not be taken to believe that my utterances upon any of the questions dealt with are infallible or incapable of 6/William McDougall being improved upon: but repeated expressions of deference and of the sense of my own uncertainty would be out of place in a semi-popular work of this character and would obscure the course of my exposition.