564 PSYCHOLOGY. 



[by tliis Locke meant identically wliat we understand by neural pro- 

 cesses] whicli, 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." * 



Hartley was more thorough iu his grasp of the prin- 

 ciple. The sensorial nerve-currents, produced when objects 

 are fully present, were for him ' vibrations,' and those which 

 produce ideas of objects in their abseuce were ' miniature 

 vibrations.' And he sums up the cause of mental associa- 

 tion in a single formula by saying : 



"Any vibrations, A, B, C, etc., by being associated together a suffi- 

 cient Number of Times, get such a Power over o, 6, c, etc., the corre- 

 sponding Miniature Vibrations, that any of the Vibrations A, when 

 impressed alone, shall be able to excite 6, c, etc., the Miniatures of *^he 

 rest. " t 



It is evident that if there be any law of neural habit 

 similar to this, the contiguities, coexistences, and succes- 

 sions, 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 



* Essay, bk. ii. chap, xxxiii. § 6. Compare Hume, who, like Locke, 

 only uses the principle to account for unreasonable and obstructive mental 

 associations : 



" 'Twould have been easy to have made an imaginary dissection of tlie 

 brain, and have shown why, upon our conception of any idea, the animal 

 spirits run into all the contiguous traces, and rouse up the other ideas that 

 are related to it. But though I have neglected any advantage which I 

 might have drawn from this topic iu explaining the relations of ideas, I am 

 afraid I must here have recourse to it. in order to account for the mistakes 

 that arise from these relations. I sliall therefore observe, that as the mind 

 is endowed with a power of exciting anj' idea it pleases ; whenever it dis- 

 patches the spirits into that region of the brain in which the idea is placed, 

 these spirits always e.xcite the idea, when they run precisely into the proper 

 traces, and rummage that cell which belongs to the idea. But as their mo- 

 tion is seldom direct, and naturally turns a little to the one side or the other: 

 for tliis reason the animal spnits, falling into the contiguous traces, pre 

 sent other related ideas in lieu of that which the mind desired at first to 

 survey. This change we are not always sen.sil)le of : but continuing still 

 the same train of thought, make use of the related idea whioli is presented 

 to us. and employ it in our reasoning, as if it were the same with what we 

 demanded. This is the cause of many mistakes and sophisms iu philoso- 

 phy; as will naturally be imagined, and as it would be easy to show, if there 

 was occasion." 



f Op. eit. prop. xi. 



