The disposition to be curious and reflective about the physical and social universe is human enough. For men, in distinction from animals, live in a world of ideas as well as in a realm of immediate reality. This world of ideas is something more than the mirror that sense-perception offers us; something less than that ultimate reality to which it seems to be a prologue and invitation. Man, in his ambition to be master of himself and of nature, looks behind the mirror, to analyze phenomena and seek causes, in order to gain control. Science, natural science, is a research for causes, that is to say, for mechanisms, which in turn find application in technical devices, organization, and machinery, in which mankind asserts its control over physical nature and eventually over man himself. Education, in its technical aspects at least, is a device of social control, just as the printing press is an instrument that may be used for the same purpose.

Sociology, like other natural sciences, aims at prediction and control based on an investigation of the nature of man and society, and nature means here, as elsewhere in science, just those aspects of life that are determined and predictable. In order to describe man and society in terms which will reveal their nature, sociology is compelled to reduce the complexity and richness of life to the simplest terms, i.e., elements and forces. Once the concepts "elements" or "forces" have been accepted, the notion of interaction is an evitable, logical development. In astronomy, for example, these elements are (a) the masses of the heavenly bodies, (b) their position, (c) the direction of their movement, and (d) their velocity. In sociology, these forces are institutions, tendencies, human beings, ideas, anything that embodies and expresses motives and wishes. In principle, and with reference to their logical character, the "forces" and "elements" in sociology may be compared with the forces and elements in any other natural science.

Ormond, in his Foundations of Knowledge,[135] gives an illuminating analysis of interaction as a concept which may be applied equally to the behavior of physical objects and persons.

The notion of interaction is not simple but very complex. The notion involves not simply the idea of bare collision and rebound, but something much more profound, namely, the internal modifiability of the colliding agents. Take for example the simplest possible case, that of one billiard ball striking against another. We say that the impact of one ball against another communicates motion, so that the stricken ball passes from a state of rest to one of motion, while the striking ball has experienced a change of an opposite character. But nothing is explained by this account, for if nothing happens but the communication of motion, why does it not pass through the stricken ball and leave its state unchanged? The phenomenon cannot be of this simple character, but there must be a point somewhere at which the recipient of the impulse gathers itself up, so to speak, into a knot and becomes the subject of the impulse which is thus translated into movement. We have thus movement, impact, impulse, which is translated again into activity, and outwardly the billiard ball changing from a state of rest to one of motion; or in the case of the impelling ball, from a state of motion to one of rest. Now the case of the billiard balls is one of the simpler examples of interaction. We have seen that the problem it supplies is not simple but very complex. The situation is not thinkable at all if we do not suppose the internal modifiability of the agents, and this means that these agents are able somehow to receive internally and to react upon impulses which are communicated externally in the form of motion or activity. The simplest form of interaction involves the supposition, therefore, of internal subject-points or their analogues from which impulsions are received and responded to.

Simmel, among sociological writers, although he nowhere expressly defines the term, has employed the conception of interaction with a clear sense of its logical significance. Gumplowicz, on the other hand, has sought to define social interaction as a principle fundamental to all natural sciences, that is to say, sciences that seek to describe change in terms of a process, i.e., physics, chemistry, biology, psychology. The logical principle is the same in all these sciences; the processes and the elements are different.

2. Classification of the Materials

The material in this chapter will be considered here under three main heads: (a) society as interaction, (b) communication as the medium of interaction, and (c) imitation and suggestion as mechanisms of interaction.

a) Society as interaction.—Society stated in mechanistic terms reduces to interaction. A person is a member of society so long as he responds to social forces; when interaction ends, he is isolated and detached; he ceases to be a person and becomes a "lost soul." This is the reason that the limits of society are coterminous with the limits of interaction, that is, of the participation of persons in the life of society. One way of measuring the wholesome or the normal life of a person is by the sheer external fact of his membership in the social groups of the community in which his lot is cast.

Simmel has illustrated in a wide survey of concrete detail how interaction defines the group in time and space. Through contacts of historical continuity, the life of society extends backward to prehistoric eras. More potent over group behavior than contemporary discovery and invention is the control exerted by the "dead hand of the past" through the inertia of folkways and mores, through the revival of memories and sentiments and through the persistence of tradition and culture. Contacts of mobility, on the other hand, define the area of the interaction of the members of the group in space. The degree of departure from accepted ideas and modes of behavior and the extent of sympathetic approach to the strange and the novel largely depend upon the rate, the number, and the intensity of the contacts of mobility.