Time-determinations apart, the facts we have run over can all be summed up in the simple statement that objects once experienced together tend to become associated in the imagination, so that when any one of them is thought of, the others are likely to be thought of also, in the same order of sequence or coexistence as before. This statement we may name the law of mental association by contiguity.[472]
I preserve this name in order to depart as little as possible from tradition, although Mr. Ward's designation of the process as that of association by continuity[473] or Wundt's as that of external association (to distinguish it from the internal association which we shall presently learn to know under the name of association by similarity)[474] are perhaps better terms. Whatever we name the law, since it expresses merely a phenomenon of mental habit, the most natural way of accounting for it is to conceive it as a result of the laws of habit in the nervous system; in other words, it is to ascribe it to a physiological cause. If it be truly a law of those nerve-centres which co-ordinate sensory and motor processes together that paths once used for coupling any pair of them are thereby made more permeable, there appears no reason why the same law should not hold good of ideational centres and their coupling-paths as well.[475] Parts of these centres which have once been in action together will thus grow so linked that excitement at one point will irradiate through the system. The chances of complete irradiation will be strong in proportion as the previous excitements have been frequent, and as the present points excited afresh are numerous. If all points were originally excited together, the irradiation may be sensibly simultaneous throughout the system, when any single point or group of points is touched off. But where the original impressions were successive—the conjugation of a Greek verb, for example—awakening nerve-tracts in a definite order, they will now, when one of them awakens, discharge into each other in that definite order and in no other way.
The reader will recollect all that has been said of increased tension in nerve-tracts and of the summation of stimuli (p. 82 ff.). We must therefore suppose that in these ideational tracts as well as elsewhere, activity may be awakened, in any particular locality, by the summation therein of a number of tensions, each incapable alone of provoking an actual discharge. Suppose for example the locality M to be in functional continuity with four other localities, K, L, N, and O. Suppose moreover that on four previous occasions it has been separately combined with each of these localities in a common activity. M may then be indirectly awakened by any cause which tends to awaken either K, L, N, or O. But if the cause which awakens K, for instance, be so slight as only to increase its tension without arousing it to full discharge, K will only succeed in slightly increasing the tension of M. But if at the same time the tensions of L, N, and O are similarly increased, the combined effects of all four upon M may be so great as to awaken an actual discharge in this latter locality. In like manner if the paths between M and the four other localities have been so slightly excavated by previous experience as to require a very intense excitement in either of the localities before M can be awakened, a less strong excitement than this in any one will fail to reach M. But if all four at once are mildly excited, their compound effect on M may be adequate to its full arousal.
The psychological law of association of objects thought of through their previous contiguity in thought or experience would thus be an effect, within the mind, of the physical fact that nerve-currents propagate themselves easiest through those tracts of conduction which have been already most in use. Descartes and Locke hit upon this explanation, which modern science has not yet succeeded in improving.
"Custom," says Locke, "settles habits of thinking in the understanding, as well as of determining in the will, and of motions in the body; all which seem to be but trains of motion in the animal spirits [by this Locke meant identically what we understand by neural processes] which, once set agoing, continue in the same steps they have been used to, which by often treading are worn into a smooth path, and the motion in it becomes easy and, as it were, natural."[476]
Hartley was more thorough in his grasp of the principle. The sensorial nerve-currents, produced when objects are fully present, were for him 'vibrations,' and those which produce ideas of objects in their absence were 'miniature vibrations.' And he sums up the cause of mental association in a single formula by saying:
"Any vibrations, A, B, C, etc., by being associated together a sufficient Number of Times, get such a Power over a, b, c, etc., the corresponding Miniature Vibrations, that any of the Vibrations A, when impressed alone, shall be able to excite b, c, etc., the Miniatures of the rest."[477]
It is evident that if there be any law of neural habit similar to this, the contiguities, coexistences, and successions, met with in outer experience, must inevitably be copied more or less perfectly in our thought. If A B C D E be a sequence of outer impressions (they may be events or they may be successively experienced properties of an object) which once gave rise to the successive 'ideas' a b c d e, then no sooner will A impress us again and awaken the a, than b c d e will arise as ideas even before B C D E have come in as impressions. In other words, the order of impressions will the next time be anticipated; and the mental order will so far forth copy the order of the outer world. Any object when met again will make us expect its former concomitants, through the overflowing of its brain-tract into the paths which lead to theirs. And all these suggestions will be effects of a material law.
Where the associations are, as here, of successively appearing things, the distinction I made at the outset of the chapter, between a connection thought of and a connection of thoughts, is unimportant. For the connection thought of is concomitance or succession; and the connection between the thoughts is just the same. The 'objects' and the 'ideas' fit into parallel schemes, and may be described in identical language, as contiguous things tending to be thought again together, or contiguous ideas tending to recur together.
Now were these cases fair samples of all association, the distinction I drew might well be termed a Spitzfindigkeit or piece of pedantic hair-splitting, and be dropped. But as a matter of fact we cannot treat the subject so simply. The same outer object may suggest either of many realities formerly associated with it—for in the vicissitudes of our outer experience we are constantly liable to meet the same thing in the midst of differing companions—and a philosophy of association that should merely say that it will suggest one of these, or even of that one of them which it has oftenest accompanied, would go but a very short way into the rationale of the subject. This, however, is about as far as most associationists have gone with their 'principle of contiguity.' Granted an object, A, they never tell us beforehand which of its associates it will suggest; their wisdom is limited to showing, after it has suggested a second object, that that object was once an associate. They have had to supplement their principle of Contiguity by other principles, such as those of Similarity and Contrast, before they could begin to do justice to the richness of the facts.