210 ANIMAL AUTOMATISM y 



just the same nature as those which occur in motor 

 nerves, when the muscles to which they are distri- 

 buted are made to contract : there is a molecular 

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

 propagated towards the brain. But the great fact 

 insisted upon by Descartes, that no likeness of 

 external things is, or can be, transmitted to the 

 mind by the sensory organs ; on the contrary, that, 

 between the external cause of a sensation and the 

 sensation, there is interposed a mode of motion of 

 nervous matter, of which the state of conscious- 

 ness is no likeness, but a mere symbol, is of the 

 profoundest importance. It is the physiological 

 foundation of the doctrine of the relativity of 

 knowledge, and a more or less complete idealism 

 is a necessary consequence of it. 



For of two alternatives one must be true. 

 Either consciousness is the function of a something 

 distinct from the brain, which we call the soul, 

 and a sensation is the mode in Avhich this soul is 

 affected by the motion of a part of the brain ; or 

 there is no soul, and a sensation is something 

 generated by the mode of motion of a part of the 

 brain. In the former case, the phenomena of the 

 senses are purely spiritual affections ; in the latter, 

 they are something manufactured by the mechan- 

 ism of the body, and as unlike the causes which 

 set that mechanism in motion, as the sound of a 

 repeater is unlike the pushing of the spring which 

 gives rise to it 



