of memorizing. The facts observed, which assist memory so
greatly, are usually relations or groups.

[Illustration: Fig. 57.--The formation of an association between two
objects by observing their grouping or relationship. (Figure text:
response of observing the Group A B, thought of Group A B)]

Evidently the observation of a group of things is a response to a
collection of stimuli, and could not originally be aroused by any one
of the stimuli alone. The same is true of observing a relationship;
the observation is a response to two things taken together, and not,
originally, to either of the two things taken alone. In spite of this,
a single one of the things may later call to mind the relationship, or
the group; that is, it arouses the response originally made to the
pair or group of stimuli. The single stimulus has been substituted for
the team that originally aroused the response. Its linkage with the
response has been so strengthened by exercise as to operate
effectively without assistance.

For example, in learning pairs of words in a "paired {405} associates
experiment", [Footnote: See p. 336.] the subject is apt to find some
relation between the words forming a pair, even though they are
supposed to be "unrelated words". When he has thus learned the pair,
either of the words in it will recall the observed relation and the
other word of the pair. Sometimes, after a long interval especially,
the relation is recalled without the other word. One subject fixed the
pair, "windy--occupy", by thinking of a sailor occupying a windy perch
up in the ropes. Some weeks later, on being given the word "windy", he
recalled the sailor on the perch, but could not get the word "occupy".
That is, he made the same response to "windy" that he had originally
made to "windy--occupy", but did not get the response completely
enough to give the second word.

In the typical cases of _association by contiguity_ when one object
reminds us of another that was formerly experienced together with it,
the law of combination comes in as just described. The two objects
were observed to be grouped or related in some way, or some such
unitary response was made to the two objects taken together, and this
response became so linked to each of the objects that later a single
one of them arouses this unitary response and recalls the other
object. In the free association test, [Footnote: See p. 380.] the
stimulus word "dimple" calls up the previou