OF THE NEW SYSTEM 327 



approve of seeking to demonstrate truths from first 

 principles: it is more useful than people think, and 

 I have often 2V put this precept in practice. Thus I com- 

 mend what you say on this point, and I would that your 

 example may lead our philosophers to think of it as they 

 ought. 



20. I will add a further reflexion, which seems to me 

 helpful in making the reality and use of my system better 

 understood. You know that M. Descartes believed in 

 the conservation of the same quantity of motion in bodies. 

 It has been shown that he was wrong in this ; but I have 

 shown that it is still true that there is conservation of 

 the same moving force, instead of which he put quantity 

 of motion. Nevertheless, he was perplexed by the 

 changes which take place in the body in consequence 

 of modifications of the soul, because they seemed to 

 break this law. But he thought he had found a way 

 out of it (which is certainly ingenious) in saying that 

 we must distinguish between motion and direction ; and 

 that the soul cannot increase nor diminish the moving 

 force, but that it changes the direction or determination 

 of the course of the animal spirits, and that it is in this 

 way that voluntary motions take place 28 . It is true that 

 he made no attempt to explain how the soul acts so as to 

 change the course of bodies, for there seems as much 

 difficulty in this as there is in saying that the soul gives 

 motion to bodies, unless with me you have recourse to 

 the pre-established harmony ; but it is to be observed 

 that there is another law of nature, which I have discovered 

 and proved, and which M. Descartes was unaware of, 

 namely, that there is conservation not only of the same 

 quantity of moving force, but also of the same quantity 



are absolutely insoluble, but because they are soluble only in 

 a certain order, which requires that philosophers should begin by 

 coming to an agreement as to the infallible mark of truth, and 

 should confine themselves to demonstrations from first principles ' 

 (E. 130 b ; G. iv. 490). 



27 E. omits ' often.' Cf. Introduction, Part ii. p. 59. 



28 See Monadology, 80, note 127, and Introduction, Partiii. p. 89. 



