in THE ORIGIN OF THE IMPRESSIONS 93 



causes of which are to be sought in the modes of 

 motion of the matter of the muscles of the wings. 



Hume, however, treats of this important topic 

 only incidentally. He seems to have had very 

 little acquaintance even with such physiology as 

 was current in his time. At least, the only passage 

 of his works, bearing on this subject, with which I 

 am acquainted, contains nothing but a very odd 

 version of the physiological views of Descartes: 



" When I received the relations of resemblance, contiguity, 

 and causation, as principles of union among ideas, without 

 examining into their causes, 'twas more in prosecution of 

 my first maxim, that we must in the end rest contented with 

 experience than for want of something specious and plausi- 

 ble which I might have displayed on that subject. "Twould 

 have been easy to have made an imaginary dissection of the 

 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 in explaining the relations of ideas, 

 I am afraid I must here have recourse to it, in order to ac- 

 count for the mistakes that arise from these relations. I 

 shall therefore observe, that as the mind is endowed 

 with the power of exciting any idea it pleases ; whenever 

 it despatches the spirits into that region of the brain in 

 which the idea is placed; these spirits always excite the 

 idea, when they run precisely into the proper traces and 

 rummage that cell which belongs to the idea. But as their 

 motion is seldom direct, and naturally turns a little to the 

 one side or to the other ; for this reason the animal spirits, 

 falling into the contiguous traces, present other related 

 ideas, in lieu of that which the mind desired at first to sur- 

 vey. This change we are not always sensible of ; but con- 



