90 NOTES TO BOOK I. 



to the appropriate Idea, namely Force, the Cause of 

 Motion, but to relations of Space and the like ; that is, 

 he introduces Geometrical instead of Mechanical Ideas. 

 It may be said that we learn little by being told that 

 Aristotle's failure in this and the like cases arose from his 

 referring to the wrong class of Ideas; or, as I have 

 otherwise expressed it, fixing his attention upon the wrong 

 aspects and relations of the facts ; since, it may be said, 

 this is only to state in other words that he did fail. But 

 this criticism is, I think, ill-founded, The account which 

 I have given is not only a statement that Aristotle, and 

 others who took a like course, did fail ; but also, that 

 they failed in one certain point out of several which are 

 enumerated. They did not fail because they neglected to 

 observe facts ; they did not fail because they omitted to 

 class facts ; they did not fail because they had not ideas 

 to reason from ; but they failed because they did not 

 take the right ideas in each case. And so long as they 

 were in the wrong in this point, no industry in collect- 

 ing facts, or ingenuity in classing them and reasoning 

 about them, could lead them to solid truth. Nor is this 

 account of the nature of their mistake without its in- 

 struction for us ; although we are not to expect to derive 

 from the study of their failure any technical rule which 

 shall necessarily guide us to scientific discovery. For 

 their failure teaches us that, in the formation of science, 

 an Errour in the Ideas is as fatal to the discovery of 

 Truth as an Errour in the Facts ; and may as completely 

 impede the progress of knowledge. I have in Books ir. 

 to x. of the Philosophy, shown historically how large a 

 portion of the progress of Science consists in the esta- 

 blishment of Appropriate Ideas as the basis of each science. 



