y have revealed, beneath differences in culture and individual
achievement, the outlines of a pervasive and relatively unchanging human
nature in which all races and individuals have a common share.

The study of human nature begins with description, but it goes on from
that point to explanation. If the descriptions which we have thus far
had of human nature are imperfect and lacking in precision, it is
equally true that the explanations thus far invented have, on the whole,
been inadequate. One reason for this has been the difficulty of the
task. The mechanisms which control human behavior are, as might be
expected, tremendously complicated, and the problem of analyzing them
into their elementary forms and reducing their varied manifestations to
precise and lucid formulas is both intricate and perplexing.

The foundation for the explanation of human nature has been laid,
however, by the studies of behavior in animals and the comparative study
of the physiology of the nervous system. Progress has been made, on the
one hand, by seeking for the precise psycho-chemical process involved in
the nervous reactions, and on the other, by reducing all higher mental
processes to elementary forms represented by the tropisms and reflex
actions.

In this, science has made a considerable advance upon common sense in
its interpretations of human behavior, but has introduced no new
principle; it has simply made its statements more detailed and exact.
For example, common sense has observed that "the burnt child shuns the
fire," that "the moth seeks the flame." These are both statements of
truths of undoubted generality. In order to give them the validity of
scientific truth, however, we need to know what there is in the nature
of the processes involved that makes it inevitable that the child should
shun the fire and the moth should seek the flame. It is not sufficient
to say that the action in one case is instinctive and in the other
intelligent, unless we are able to give precise and definite meanings to
those terms; unless, in short, we are able to point out the precise
mechanisms through which these reactions are carried out. The following
illustration from Loeb's volume on the comparative physiology of the
brain will illustrate the distinction between the common sense and the
more precise scientific explanation of the behavior in man and the lower
animals.

It is a well-known fact that if an ant be removed from a nest
a