rom 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,