ASSOCIATION. 568 



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 in- 

 creased tension in nerve-tracts and of the summation of 

 stimuli (p. 82 ff.). We must therefore supj)ose 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 simi- 

 larly 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 com- 

 pound effect on M may be adequate to its full arousal. 



The psychological laiv of association of objects thought of 

 through their previous contiguity in thought or experience 

 would thus he an effect, ivithin the mind, of the physical fact 

 that nerve-currents propagate themselves easiest through those 

 tracts of conduction ivhich have been already most in use. Des- 

 cartes and Locke hit upon this explanation, which modern 

 science has not yet succeeded in improving. 



"Custom," says Locke, •' settles habits of thinking in the under- 

 standing, as well as of determinin.s: in the will, and of motions in the 

 body ; all which seem to be but trains of motion in the animal spirits 



