IT is by the pleasure of exertion, and the pain of inexertion, that we are
roused from that indolence, into which . . . we otherwise might sink:
as we are roused, in like manner, by the pleasure of food, and the pain
of hunger, we take the aliment that is necessary for our individual
sustenance; and though the mere aliment is, indeed, more important
for life, it is not more important for happiness than the pleasure of
activity which calls and forces us from slothful repose.

Thomas Brown, Lectures on the Philosophy
of the Human Mind, 1822

We have been occupied in exploring the principle of rein-
forcement and the manner in which the environment con-
trols organisms by way of stimuli. From this single starting
point, we have been able to take large strides in understand-
ing why men and lower animals behave as they do. Yet
students of psychology, in past times and present, have felt or
known that a description of behavior would be incomplete
without taking into account another kind of controlling fac-
tor which today we call motivation.

Common experience reveals the existence of this factor so
vividly that men everywhere have evolved a vocabulary and
set of ideas for explaining and speaking of it. Growing up in
a social community as we do, we are taught the prevailing
words and concepts. These seem consequently, to be right,
natural, and but common sense. Unhappily, there are few
areas in psychology where popular notions contain a more
alluring blend of correct and incorrect observations, of valid
and biased thinking, of wise and foolish conclusions. Our

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