Since, then, the social aggregate has a collective mental life, which is not merely the sum of the mental lives of its units, it may be contended that a society not only enjoys a collective mental life but also has a collective mind or, as some prefer to say, a collective soul. The tasks of Group Psychology are, then, to examine the conception of the collective or group mind, in order to determine whether and in what sense this is a valid conception; to display the general principles of collective mental life which are incapable of being deduced from the laws of the mental life of isolated individuals; to distinguish the principal types of collective mental life or group mind; to describe the peculiarities of those types and as far as possible to account for them. More shortly, Group Psychology has, first, to establish the general principles of group life (this is general collective psychology); secondly, it has to apply these principles in the endeavour to understand particular examples of group life. Group Psychology, thus conceived, meets at the outset a difficulty which stands in the way of every attempt of psychology to leave the narrow field of highly abstract individual psychology. It finds the ground already staked out and occupied by the representatives of another science, who are inclined to resent its intrusion as an encroachment on their rights. The science which claims to have occupied the field of Group Psychology is Sociology; and it is of some importance that the claims of these sciences should be reconciled, so that they may live and work harmoniously together. I have no desire to claim for Group Psychology the whole province of Sociology. As I conceive it, that province is much wider than that of Group Psychology. Sociology is essentially a science which has to take a comprehensive and synthetic view of the life of mankind, and has to accept and make use of the conclusions of many other more special sciences, of which psychology, and especially Group Psychology, is for it perhaps the most important. But other special sciences have very important if less intimate contributions to make to it. Thus, if it be true that great civilisations have decayed owing to changes of climate of their habitats, or owing to the introduction of such diseases as malaria into them, then Climatology and Epidemiology have their contributions to make to Sociology. If peculiarities of diet or the crossing of racial stocks may profoundly affect the vigour of peoples, Physiology must have its say. General biology and the science of Genetics are bringing to light much that must be incorporated in Sociology. Economics, although needing to be treated far more psychologically than it commonly has been, has its special contribution to make. These are only a few illustrations of the fact that the field of Sociology is very much wider and more general than that of Group Psychology, however important to it the conclusions of the narrower science may be.