§ 31. =The Association of Ideas.=—The doctrine of the ‘association of ideas’ is one of the oldest and most influential in the history of psychology. It begins, in a somewhat casual way, with Aristotle. Suppose, Aristotle says, that we are trying to recall something that has slipped our mind; what do we ordinarily do? We hunt through a number of things, beginning with something that is like what we want to recall, or contrary to it, or that was next it in time, or adjacent to it in space. These other things, the like, the contrary, the just before or just after, the adjoining, have the power to suggest what we have forgotten. Aristotle gives the impression that everybody acts in this way, as a matter of course; and no doubt his hearers acquiesced; for the statement sounds reasonable. We want, for instance, to remember a certain picture that we saw ten years ago: how do we set to work? We start from something like it: ‘I remember that it reminded me of Van Eyck’; or from something opposite: ‘I remember smiling to think how a Venetian would have treated it’; or from something next it in time: ‘I remember coming to it after three whole hours of Dutch _genre_’; or from something next it in space: ‘I remember that it hung beside a Mabuse portrait.’ Seeing how natural and obvious such remarks are, we can understand that Aristotle’s single sentence had tremendous consequences for psychology. It foreshadowed the four ‘laws of the association of ideas,’ _the laws of similarity, of contrast, of succession in time, and of coexistence in space_. According to the doctrine of association, one idea ‘calls up’ another _because_ it is like that other, or contrasts with it, or was next to it in time or space; likeness and difference, succession and adjacency, somehow give an idea the power to recall, and render it liable in its turn to be recalled. The four laws thus represent an attempt to _explain_ the course of our ideas, and for that reason they have always appealed to common sense.