92 HUME III 



But, as this latter conclusion is contrary to evident experience, 

 and as 'tis possible we may have a like experience in the opera- 

 tions of the mind, and may perceive a constant conjunction of 

 thought and motion, you reason too hastily when, from the 

 mere consideration of the ideas, you conclude that 'tis impossible 

 motion can ever produce thought, or a different position of 

 parts give rise to a different passion or reflection. Nay, 'tis not 

 only possible we may have such an experience, but 'tis certain 

 we have it ; since every one may perceive that the different 

 dispositions of his body change his thoughts and sentiments. 

 And should it be said that this depends on the union of soul 

 and body, I would answer, that we must separate the question 

 concerning the substance of the mind from that concerning the 

 cause of its thought ; and that, confining ourselves to the latter 

 question, we find, by the comparing their ideas, that thought 

 and motion are different from each other, and by experience, 

 that they are constantly united ; which, being all the circum- 

 stances that enter into the idea of cause and effect, when applied 

 to the operations of matter, we may certainly conclude that 

 motion may be, and actually is, the cause of thought and per- 

 ception." (I. pp. 314 316.) 



The upshot of all this is, that the " collection of 

 perceptions/' which constitutes the mind, is really 

 a system of effects, the causes of which are to be 

 sought in antecedent changes of the matter of the 

 brain, just as the "collection of motions," which 

 we call flying, is a system of effects, the 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 



