To define exactly the relations of the several special sciences is a task which can never be completely achieved so long as these sciences continue to grow and change. It is a peculiarly difficult task in respect of the biological sciences, because we have not yet reached general agreement as to the fundamental conceptions which these sciences should employ. To illustrate this difficulty I need only refer to a recent symposium of the Aristotelian Society in which a number of distinguished philosophers and biologists discussed the question “Are physical, biological and psychological categories irreducible?” The discussion revealed extreme differences of opinion, and failed to bring the disputants nearer to a common view. The difficulty is still greater in respect of the human sciences—anthropology, psychology, ethics, politics, economics, sociology, and the rest; and it is not to be hoped that any general agreement on this difficult question will be reached in the near future. Yet it seems worth while that each writer who aspires to break new ground in any part of this field of inquiry should endeavour to make clear to himself and others his conception of the relations of that part to the rest of the field. It is, then, in no dogmatic spirit, or with any belief in the finality of the position assigned to my topic, that I venture the following definition of the province of psychology with which this book is concerned. I have chosen the title, “The Group Mind,” after some hesitation in favour of the alternative, “Collective Psychology.” The latter has the advantage that it has already been used by several continental authors, more especially French and Italian psychologists. But the title I have chosen is, I think, more distinctively English in quality and denotes more clearly the topic that I desire to discuss.