The concepts upon which this explanation of society rests is _homogeneity_. If animals or human beings act under all circumstances in the same way, they will act or seem to act, as if they had a common purpose. If everybody follows the crowd, if everyone wears the same clothes, utters the same trite remarks, rallies to the same battles cries and is everywhere dominated, even in his most characteristically individual behavior, by an instinctive and passionate desire to conform to an external model and to the wishes of the herd, then we have an explanation of everything characteristic of society--except the variants, the nonconformists, the idealists, and the rebels. The herd instinct may be an explanation of conformity but it does not explain variation. Variation is an important fact in society as it is in nature generally.
Homogeneity and like-mindedness are, as explanations of the social behavior of men and animals, very closely related concepts. In "like response to like stimulus," we may discern the beginning of "concerted action" and this, it is urged, is the fundamental social fact. This is the "like-mindedness" theory of society which has been given wide popularity in the United States through the writings of Professor Franklin Henry Giddings. He describes it as a "developed form of the instinct theory, dating back to Aristotle's aphorism that man is a political animal."
Any given stimulus may happen to be felt by more than one organism, at the same or at different times. Two or more organisms may respond to the same given stimulus simultaneously or at different times. They may respond to the same given stimulus in like or in unlike ways; in the same or in different degrees; with like or with unlike promptitude; with equal or with unequal persistence. I have attempted to show that in like response to the same given stimulus we have the beginning, the absolute origin, of all concerted activity--the inception of every conceivable form of co-operation; while in unlike response, and in unequal response, we have the beginning of all those processes of individuation, of differentiation, of competition, which in their endlessly varied relations to combination, to co-operation, bring about the infinite complexity of organized social life.[36]