The law of exercise, with all its sub-laws, is certainly fundamental and universal; it is always in operation whenever anything is learned; and yet, just by itself, it goes only halfway towards accounting for learned reactions. For a reaction to be exercised, it must be made, and the law of exercise presupposes that it is made, and does not attempt to account for its being made in the first place.
The law of exercise does not cover the formation of new linkages, but only the strengthening of linkages that are already working. It does not explain the attachment of a response to some other than its natural stimulus, nor the combination, of responses into a higher unit, nor the association of two facts so that one later recalls the other. We learn by doing, but how can we do anything new so as to start to learn? We learn by observing combinations of facts, but how in the first place do we combine the facts in our minds?
How, for example, can we learn to respond to the sight of the person by saying his name? Evidently, by exercising this linkage of stimulus and response. But how did we ever make a start in responding thus, since there is nothing about the person's looks to suggest his name? The name came to us through the ear, and the face by way of the eye; and if we repeated the name, that was a response to the auditory stimulus and not to the visual. How has it come about, then, that we later respond to the visual stimulus by saying the name?
In short, the more seriously 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, establishes an indirect contiguity between the two things, a [{396}] sort of contiguity bridge between them. One thing has the parts or characteristics, A B X Y, and the similar thing has the parts or characteristics, C D X Y; and thus X Y, when seen in the second thing, call up A B, with which they are contiguous in the first thing.