ences, 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,
f