ANIMAL AUTOMATISM. 215 



ously moved. . . . And we must take care not to imagine that, in 

 order to feel, the soul needs to behold certain images sent by the 

 objects of sense to the brain, as our philosophers commonly sup- 

 pose ; or, at least, we must conceive these images to be something 

 quite different from what they suppose them to be. For, as all 

 they suppose is that these images ought to resemble the objects 

 which they represent, it is impossible fur them to show how they 

 can be formed by the objects received by the organs of the external 

 senses and transmitted to the brain. And they have had no reason 

 for supposing the existence of these images except this ; seeing that 

 the mind is readily excited by a picture to conceive the object which 

 is depicted, they have thought that it must be excited in the same 

 way to conceive those objects which affect our senses by little pic- 

 tures of them formed in the head; instead of which we ought to 

 recollect that there are many things besides images which may ex- 

 cite the mind, as, for example, sjgns^and words, which have not the 

 least resemblance to the objects which they signify."* 



Modern physiology amends Descartes' conception of 

 the mode of action of sensory nerves in detail, by show- 

 ing that their structure is the same as that of motor 

 nerves ; and that the changes which take place in them, 

 when the sensory organs with which they are connected 

 are excited, are of just the same nature as those which 

 occur in motor nerves, when the muscles to which they 

 are distributed are made to contract : there is a molecu- 

 lar change which, in the case of the sensory nerve, is 

 propagated towards the brain. But the great fact in- 

 sisted upon by Descartes, that no likeness of external 



* Locke (Human Understanding, Book II., chap, viii. 37) uses Des- 

 cartes' illustration for the samo purpose, and warns us that " most of the 

 ideas of sensation are no more the likeness of something existing without 

 us than the names that stand for them are the likeness of our ideas, which 

 yet, upon hearing, they arc apt to excite in us," a declaration which paved 

 the way for Berkeley. 

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