Association of ideas, or mental association, is a process by which representations arise in consciousness, and also for a principle put forward by an important historical school of thinkers to account generally for the succession of mental phenomena. It is used mostly in the history of philosophy and of psychology. One idea was thought to follow another in consciousness if it were associated by some principle. The three commonly asserted principles of association were similarity, contiguity, and contrast, numerous others had been added by the nineteenth century. By the end of the nineteenth century physiological psychology was so altering the approach to this subject that much of the older associationist theory was rejected.
Everyday observation of the association of one idea or memory with another gives a face validity to the notion. In addition, the notion of association between ideas and behavior gave some early impetus to behaviorist thinking. The core ideas of associationist thinking recur in some recent thought on cognition, especially consciousness.
The associationist theory is anticipated in Plato's Phaedo, as part of the doctrine of anamnesis. The idea of Simmias is recalled by the picture of Simmias (similarity) and that of a friend by the sight of the lyre on which he played (contiguity). But Aristotle is credited with originating associationist thinking based on this passage:
"When, therefore, we accomplish an act of reminiscence, we pass through a certain series of precursive movements, until we arrive at a movement on which the one we are in quest of is habitually consequent. Hence, too, it is that we hunt through the mental train, excogitating from the present or some other, and from similar or contrary or coadjacent. Through this process reminiscence takes place. For the movements are, in these cases, sometimes at the same time, sometimes parts of the same whole, so that the subsequent movement is already more than half accomplished."
The passage is obscure, but it does at all events indicate the various principles commonly termed contiguity, similarity, and contrast. Similar principles are stated by Zeno the Stoic, by Epicurus (see Diogenes Laertius vii. § 52, x. § 32), and by St Augustine of Hippo (Confessions, x. c. 19). Aristotle's doctrine received a more or less intelligent expansion and illustration from the ancient commentators and the schoolmen, and in the still later period of transition from the age of scholasticism to the more modern philosophy, prolonged in the works of some writers far into the 17th century. William Hamilton adduced not a few philosophical authorities who gave prominence to the general fact of mental association - the Spanish philosopher Ludovicus Vives (1492-1540) especially being exhaustive in his account of memory.