eriously we take the law of exercise, the more we
feel the need of a supplementary law to provide for the first making
of a reaction that then, by virtue of exercise, is strengthened.

This is the problem that occupied the older writers on psychology when
they dealt with "association"; and their solution of the problem was
formulated in the famous "laws of association". The laws of
association were attempts to explain how facts got associated, so that
later one could recall another.

These laws have a long history. From Aristotle, the ancient Greek who
first wrote books on psychology, there came down to modern times four
laws of association. Facts become associated, according to Aristotle,
when they are {395} contiguous (or close together) in space, or when
they are contiguous in time, or when they resemble each other, or when
they contrast with each other. The psychologists of the earlier modern
period, in the eighteenth and first part of the nineteenth centuries,
labored with very good success to reduce these four laws to one
comprehensive law of association. Contiguity in space and in time were
combined into a law of association by _contiguity in experience_,
since evidently mere physical contiguity between two objects could
establish no association between them in any one's mind except as he
experienced them together.

Association by Similarity

Continuing their simplification of the laws of association, these
older psychologists showed that resemblance and contrast belonged
together, since to be similar things must have something in common,
and to be contrasted also two things must have something in common.
You contrast north with south, a circle and a square, an automobile
and a wheelbarrow; but no one thinks of contrasting north with a
circle, south with an automobile, or a square and a wheelbarrow,
though these pairs are more incongruous than the others. Things that
are actually associated as contrasting with each other have something
in common; and therefore association by contrast could be included
under association by similarity. Thus the four laws had been reduced
to two, association by contiguity and association by similarity.

The final step in this reduction was to show that association by
similarity was a special case of association by contiguity. To be
similar, two things must have something in common, and this common
part, being contiguous with the remainder of each of the two things,
establish