222 HABIT AND INTELLIGENCE. [chap. 



single step which has probably been made in the pro- 

 gress of science since the time of Newton consists in 

 the discovery of the laws of heat, and their establish- 

 ment as a particular case of the more general theory of 

 force. 



But, like all mere illustrations, this comparison of 



scientific progress to the progress of a picture towards 



completion is only partially applicable ; and it is not 



The truth rigidly true, but only an approximation to the truth, to 



above is ^^Y ^^^^^ ^^^ historical order in which the sciences have 



only ai> been successively evolved corresponds to the logical order 



' of their arrangement, beginning with the most general and 



simple subjects, and going on to the more special and 



complex. The logical order of the sciences throws a real 



and valuable light on their history. But it is, after all, 



only a logical formula ; a logical formula will be always 



misleading if it is mistaken for actual historical fact ; and 



this formula only explains half the facts. It is true that 



the progress of science is from more general truths to 



more special ones ; but it is also true that it consists, at 



the same time, in the discovery of special facts and in 



The generalizing from them. Thus the progress of science is 



progress of twofold, and in two directions at once ; it is at once down- 



seience is ' _ ... . 



twofold, ward, or deductive, consisting in the application of known 

 and^° ^^"^ ^^'^'^ ^0 ^*^^^ cases ; and upward, or inductive, consisting in 

 inductive, the discovery of new laws by a process of generalizing 

 from facts.^ These two processes of discovery, the deduc- 

 Mathe- tive and the inductive, sometimes go on apart. In mathe- 

 deductive ^^^tics, all is deduction ; in chemistry, all until now has 

 chemistry been induction, except the deductive element which enters 

 inductive : i^^o ^^^ relations of chemistry with the laws of electricity 

 and heat. But at other times induction and deduction co- 

 operate, as in the instance just mentioned of the ascertain- 

 ment of the theory of heat as a particular case of the 

 the general theory of force and energy. The laws of heat 



heat i5 ° were ascertained inductively, by experiment, and by gene- 

 both. ralized inference from a variety of experiments ; and the 



1 See " The Genesis of Science," in vol. i. of Herbert Spencer's collected 

 Essays. 



