94 THE LIMITATIONS OF SCIENCE 



over such a detail, when the entire conception is not 

 light nor anything resembling light. Descartes may, 

 perhaps, prove that this tendency to move proceeds 

 in straight lines and would be reflected and refracted 

 in agreement with the laws of light; that the inter- 

 stellar spaces would be transparent; that terrestrial 

 bodies would vary from transparency to opacity ac- 

 cording to the greater proportion and complexity of 

 matter of the third kind in their composition; but the 

 mechanism of all this is far more obscure and com- 

 plicated than the phenomena it attempts to elucidate. 



This theory of light advanced in the Principia was, 

 in a sense, supported by the rather meager experimental 

 knowledge then existing, and seems at first plausible. 

 But examined critically, and with the mind no longer 

 awed, or perhaps hypnotized, by Descartes's sweep of 

 imagination and his power of making words seem to 

 express clear ideas when they really do not, his cause 

 of light is found to be as arbitrary as the fiat lux of a 

 God, and its effects mere confusion. In fact, it is the 

 unavoidable weakness of any such hypothesis, and a 

 very irritating and tantalizing weakness, that the words 

 used apparently express things we can understand, and 

 yet when we try to visualize these things, stripped of 

 technical and intricate verbiage, the mind has received 

 no clear impression. 



And here we have a splendid example of an hypoth- 

 esis, whose foundations now seem ridiculous; whose 



