erious occasion. What we now have before us is one of
the great outstanding problems of psychology, a problem that has come
down through the ages, with succeeding generations of psychological
thinkers contributing of their best to its solution; and our task is
to attack this problem afresh in the light of modern knowledge of the
facts of learning and memory. We wish to gather up the threads from
the three preceding chapters, which have detailed many facts regarding
learned reactions of all sorts, and see whether we cannot summarize
our accumulated knowledge in the form of a few great laws. We wish
also to relate our laws to what is known of the brain machinery.

The Law of Exercise

Of one law of learning, we are perfectly sure. There is no doubt that
the exercise of a reaction strengthens it, makes it more precise and
more smooth-running, and gives it an advantage over alternative
reactions which have not been exercised. Evidence for these statements
began to appear as soon as we turned the corner into this part of our
subject, and has accumulated ever since. This law is sometimes called
the "law of habit", but might better be called the "law of improvement
of a reaction through exercise", or, more briefly, the "law of
exercise".

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The law of exercise is very broad in its scope, holding good of life
generally and not alone of mental life. Exercise of a muscle develops
the muscle, exercise of a gland develops the gland; and, in the same
way, exercise of a mental reaction strengthens the machinery used in
making that reaction.

Let us restate the law in terms of stimulus and response. _When a
given stimulus arouses a certain response, the linkage between that
stimulus and that response is improved by the exercise so obtained_,
and thereafter the stimulus arouses the response more surely, more
promptly, more strongly than before.

Under the law of exercise belong several _sub-laws_ already familiar
to us.

1. The law of _frequency_ refers to the cumulative effect of repeated
exercise. The practice curve gives a picture of this sub-law, showing
how improvement with repeated exercise of a performance is rapid at
first and tapers off into the physiological limit, beyond which level
more repetition cannot further improve the performance. The
superiority of "spaced study" over unspaced means that exercise is
more effective when rest periods intervene between the periods of
exercise; as this is notoriously tru