xiv.] ON DESCARTES &quot;DISCOURSE.&quot; 295 



far as that title is rightly applicable to the doctrine that we 

 have no knowledge of any thinking substance, apart from 

 extended substance ; and that thought is as much a function of 

 matter as motion is. Thus we arrive at the singular result that, 

 of the two paths opened up to us in the &quot; Discourse upon 

 Method,&quot; the one leads, by way of Berkeley and Hume, to Kant 

 and Idealism ; while the other leads, by way of De La Mettrie 

 and Priestley, to modern physiology and Materialism. 1 Our 

 stem divides into two main branches, which grow in opposite 

 ways, and bear flowers which look as different as they can well 

 be. But each branch is sound and healthy, and has as much 

 life and vigour as the other. 



If a botanist found this state of things in a new plant, I 

 imagine that he might be inclined to think that his tree was 

 monoecious that the 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 comple 

 mentary, not antagonistic ; 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 opera 

 tions of which will, sooner or later, be explained on physical 

 principles. I believe that we shall, sooner or later, arrive at a 

 mechanical equivalent of consciousness, 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 



1 Bouillier, into whose excellent &quot; History of the Cartesian Philosophy &quot; 

 I had not looked when this passage was written, says, very justly, that 

 Descartes &quot; a merite&quot; le titre de pere de la physique, aussi bien que celui de 

 pere de la metaphysique moderne &quot; (t. i. p. 197). See also Kuno Fischer s 

 &quot; Geschichte der neuen Philosophic,&quot; Bd. i. ; and the very remarkable work 

 of Lange, &quot;Geschichte des Materialismus.&quot; A good translation of the 

 latter would be a great service to philosophy in England. 



