Group Psychology itself consists properly of two parts, that which is concerned to discover the most general principles of group life, and that which applies these principles to the study of particular kinds and examples of group life. The former is logically prior to the second; though in practice it is hardly possible to keep them wholly apart. The present volume is concerned chiefly with the former branch. Only when the general principles of group life have been applied to the understanding of particular societies, of nations and the manifold system of groups within the nation, will it be possible for Social Psychology to return upon the individual life and give of it an adequate account in all its concrete fulness. The nature of Group Psychology may be illustrated by reference to Herbert Spencer’s conception of sociology. Spencer pointed out that, if you set out to build a stable pile of solid bodies of a certain shape, the kind of structure resulting is determined by the shapes and properties of these units, that for example, if the units are spheres, there are only very few stable forms which the pile can assume. The same is true, he said, of such physical processes as crystallisation; the form and properties of the whole or aggregate are determined by the properties of the units. He maintained with less plausibility that the same holds good of animal and vegetable forms and of the elements of which they are composed. And he went on to argue that, in like manner, the structure and properties of a society are determined by the properties of the units, the individual human beings, of which it is composed. This last proposition is true in a very partial sense only. For the aggregate which is a society has, in virtue of its past history, positive qualities which it does not derive from the units which compose it at any one time; and in virtue of these qualities it acts upon its units in a manner very different from that in which the units as such interact with one another. Further, each unit, when it becomes a member of a group, displays properties or modes of reaction which it does not display, which remain latent or potential only, so long as it remains outside that group. It is possible, therefore, to discover these potentialities of the units only by studying them as elements in the life of the whole. That is to say, the aggregate which is a society has a certain individuality, is a true whole which in great measure determines the nature and the modes of activity of its parts; it is an organic whole. The society has a mental life which is not the mere sum of the mental lives of its units existing as independent units; and a complete knowledge of the units, if and in so far as they could be known as isolated units, would not enable us to deduce the nature of the life of the whole, in the way that is implied by Spencer’s analogies.