PSYCHOLOGICAL SCIENCE 87 



spontaneous disintegration of radioactive elements 

 into simpler atoms shows that, in such matters, 

 certainty cannot be reached. We can conceive of a 

 woild where all chemical elements are unstable, 

 where energy is not constant, or where masses do 

 not gravitate; but we cannot conceive of a world 

 in which two and two do not make four, or where 

 things equal to the same thing are not equal to each 

 other. 



Such logical principles, then, are known to us by 

 intuitive apprehension. They have often been 

 treated as laws of thought, and in this view are a 

 subject of psychology as well as the root of logic and 

 pure mathematics. 



But, besides being laws of thought, in accordance 

 with which we are compelled to think, they are true 

 in the external world. Two shillings and two shillings 

 in actual experience always make four shillings, in 

 accordance with the general principle that two and 

 two make four. Hence, some philosophers hold that 

 the laws of thought give us certain knowledge about 

 a real world, the world of universals, and are laws of 

 nature rather than laws of thought. But others 

 would hold that the fact that our laws of thought are 

 in accordance with the particular instances that 

 happen in nature is a matter of experience, and might 

 conceivably fail us. On this view it seems better to 

 take such self-evident logical principles as a founda- 

 tion for our purely ideal and universal constructions 



