NOTES TO BOOK I. 91 



Of the two main processes by which science is con- 

 structed, as stated in Book xi. of that work, namely the 

 Explication of Conceptions and the Colligation of Facts, the 

 former must precede the latter. In Book xn. chap. 5, of 

 the Philosophy, I have stated the maxim concerning ap- 

 propriate Ideas in this form, that the Idea and the Facts 

 must oe homogeneous. 



When I say that the failure of the Greeks in physical 

 science arose from their not employing appropriate Ideas 

 to connect the facts, I do not use the term "appropriate" 

 in a loose popular sense ; but I employ it as a somewhat 

 technical term, to denote the appropriate Idea, out of that 

 series of Ideas which have been made (as I have shown 

 in the Philosophy) the foundation of sciences; namely 

 Space, Time, Number, Cause, Likeness, Substance, and 

 the rest. It appears to me just to say that Aristotle's 

 failure in his attempts to deal with problems of equili- 

 brium, arose from his referring to circles, velocities, no- 

 tions of natural and unnatural, and the like, conceptions 

 depending upon Ideas of Space, of Nature, &c. which are 

 not appropriate to these problems, and from his missing 

 the Idea of Mechanical Force or Pressure, which is the 

 appropriate Idea. 



I give this, not as an account of all failures in attempts 

 at science, but only as the account of such radical and 

 fundamental failures as this of Aristotle ; who, with a 

 knowledge of the facts, failed to connect them into a 

 really scientific view. If I had to compare rival theories 

 of a more complex kind, I should not necessarily say that 

 one involved an appropriate Idea and the other did not, 

 though I might judge one to be true and the other to be 

 false. For instance, in comparing the emissive and the 



