OF NATURAL PHILOSOPHY. 87 



which we receive of the nature of force, from our 

 own effort and our sense of fatigue, is quite different 

 from that which we obtain of it from seeing the 

 effect of force exerted by others in producing mo- 

 tion. Were there no such thing as motion, had 

 we been from infancy shut up in a dark dungeon, 

 and every limb encrusted with plaster, this internal 

 consciousness would give us a complete idea of 

 force; but when set at liberty, habit alone would 

 enable us to recognize its exertion by its signal, 

 motion, ana that only by finding that the same 

 action of the mind which in our confined state en- 

 ables us to fatigue and exhaust ourselves by the 

 tension of our muscles, puts it in our power, when 

 at liberty, to move ourselves and other bodies. But 

 how obscure is our knowledge of the process going 

 on within us in the exercise of this important privi- 

 lege, in virtue of which alone we act as direct causes, 

 we may judge from this, that when we put any limb 

 in motion, the seat of the exertion seems to us to 

 be in the limb, whereas it is demonstrably no such 

 thing, but either in the brain or in the spinal 

 marrow; the proof of which is, that if a little fibre, 

 called a nerve, which forms a communication between 

 the limb and the brain, or spine, be divided in any 

 part of its course, however we may make the effort, 

 the limb will not move. 



(78.) This one instance of the obscurity which 

 hangs about the only act of direct causation of 

 which we have an immediate consciousness, will 

 suffice to show how little prospect there is that, 

 in our investigation of nature, we shall ever be able 

 to arrive at a knowledge of ultimate causes, and will 



