iv DESCARTES' DISCOURSE ON METHOD 191 



flowers were of different sexes, and that, so far 

 from setting up a barrier between the two branches 

 of the tree, the only hope of fertility lay in 

 bringing them together. I may be taking too 

 much of a naturalist's view of the case, but I 

 must confess that this is exactly my notion of 

 what is to be done with metaphysics and physics. 

 Their differences are complementary, not antagon- 

 istic ; and thought will never be completely fruitful 

 until the one unites with the other. Let me try 

 to explain what I mean. I hold, with the 

 Materialist, that the human body, like all living 

 bodies, is a machine, all the operations of which 

 will, sooner or later, be explained on physical 

 principles. I believe that we shall, sooner or 

 later, arriva at a mechanical equivalent of con- 

 sciousness, just as we have arrived at a mechanical 

 equivalent of heat. If a pound weight falling 

 through a distance of a foot gives rise to a definite 

 amount of heat, which may properly be said to be 

 its equivalent; the same pound weight falling 

 through a foot on a man's hand gives rise to a 

 definite amount of feeling, which might with 

 equal propriety be said to be its equivalent in 

 consciousness. 1 And as we already know that 

 there is a certain parity between the intensity of 

 a pain and the strength of one's desire to get rid 



1 For all the qualifications which need to be made here, I 

 refer the reader to the thorough discussion of the nature of the 

 relation between nerve- action and consciousness in Mr. Herbert 

 Spencer's Principles of Psychology, p. 115 ct seq. 



