INFLUENCE ON RECENT SCIENCE 81 



and to disarm scientific criticism, his postulates he de- 

 clared to be fictions, but in some way without depart- 

 ing from correct mathematical deductions, his conclu- 

 sions were similar to experience. 



It would be difficult to understand from these con- 

 tradictory statements what was Descartes's real opinion 

 of his scheme. But he has this answer in another part 

 of his Principia, an answer which carries conviction. 

 Since we have an idea of perfection, we have also 

 the certainty of the existence of God, or an all-perfect 

 Being. And because this God is justice itself, if the 

 principles laid down by him are evident, if the con- 

 sequences are founded on mathematical laws, and if 

 the deductions agree with experience, it would be 

 ascribing injustice to God to believe that the causes of 

 the effects are false. Now observe how far Descartes 

 believes he has fulfilled these conditions: " je ne pense 

 pas qu'on doive recevoir d'autres principes en la 

 Physique, ni mesme qu'on ait raison d'en souhaiter 

 d'autres, que ceux qui sont icy expliquez." 



Descartes has recorded for us how he arrived at the 

 postulates from which he developed his natural laws. 

 Having previously laid aside preconceived ideas, he 

 found that to doubt is the first and only means of 

 knowledge; we can doubt the existence of everything 

 except that which doubts, therefore that which doubts 

 or thinks, exists. Whence he derives the principle 

 that our thoughts and the things we have an idea of, 



