It is a notorious fact that, when a number of men think and feel and act together, the mental operations and the actions of each member of the group are apt to be very different from those he would achieve if he faced the situation as an isolated individual. Hence, though we may know each member of a group so intimately that we can, with some confidence, foretell his actions under given circumstances, we cannot foretell the behaviour of the group from our knowledge of the individuals alone. If we would understand and be able to predict the behaviour of the group, we must study the way in which the mental processes of its members are modified in virtue of their membership. That is to say, we must study the interactions between the members of the group and also those between the group as a whole and each member. We must examine also the forms of group organisation and their influence upon the life of the group. Groups differ greatly from one another in respect of the kind and degree of organisation they possess. In the simplest case the group has no organisation. In some cases the relations of the constituent individuals to one another and to the whole group are not in any way determined or fixed by previous events; such a group constitutes merely a mob. In other groups the individuals have certain determinate relations to one another which have arisen in one or more of three ways: (1) Certain relations may have been established between the individuals, before they came together to form a group; for example, a parish council or a political meeting may be formed by persons belonging to various definitely recognised classes, and their previously recognised relations will continue to play a part in determining the collective deliberations and actions of the group; they will constitute an incipient organisation. (2) If any group enjoys continuity of existence, certain more or less constant relations, of subordination, deference, leadership and so forth, will inevitably become established between the individuals of which it is composed; and, of course, such relations will usually be deliberately established and maintained by any group that is united by a common purpose, in order that its efficiency may be promoted. (3) The group may have a continued existence and a more or less elaborate and definite organisation independently of the individuals of which it is composed; in such a case the individuals may change while the formal organisation of the group persists; each person who enters it being received into some more or less well-defined and generally recognised position within the group, which formal position determines in great measure the nature of his relations to other members of the group and to the group as a whole.