AS A MENTAL OPERATION 9 



the uniformities of Nature is not the deductive method 

 of descending at once from principles easily found, but 

 the inductive method of gradually ascending to princi- 

 ples. What Aristotle did for the deductive method' of 

 the mathematician Bacon did for the inductive method 

 of the physicist. Aristotle, it is true, had recognized 

 experience and induction. But what is due to Badbn is 

 the discrimination between the ordinary and the scien- 

 tific use of these mental operations. While ordinary 

 men content themselves with isolated experiences and 

 inductions from a few instances, the scientific man 

 requires system in both. He must, in the first place, 

 use experiment as well as observation, because Nature 

 yields her secrets when tortured and put into artificial 

 conditions by man. He must make his experience 

 quantitative. His experience must be registered in 

 writing, because it consists of too many facts for mere 

 memory. It must be communicated to others and 

 be the result of associated labours, of a society, of 

 a royal society. Secondly, as he passes from ex- 

 perience to induction, the man of science must tabu- 

 late or arrange his instances according as they are 

 instances where the thing investigated is present, in- 

 stances where it is absent in similar circumstances, 

 instances where it varies in degree. When his in- 

 stances have been tabulated in these three ways, he 

 must first use elimination, by excluding anything which 

 is absent when the thing investigated is present, any- 

 thing which is present when it is absent, anything which 

 does not vary concomitantly with it. Then, at last, he 

 will reach the affirmative induction that whatever is 

 constantly present, absent, and varying with the thing 

 investigated is always essential to this thing. This 

 systematic induction from systematic experience is what 



