TRAINS OF REASONING. 289 



The opposition is not between the terms Deductive 

 and Inductive, but between Deductive and Experi- 

 mental. A science is Experimental, in proportion as 

 every new case, which presents any peculiar features, 

 stands in need of a new set of observations and expe- 

 riments, a fresh induction. It is Deductive, in pro- 

 portion as it can draw conclusions, respecting cases 

 of a new kind, by processes which bring those cases 

 under old inductions; by ascertaining that cases which 

 cannot be observed to have the requisite marks, have, 

 however, marks of those marks. 



We can now, therefore, perceive what is the 

 generic distinction between sciences which can be 

 made Deductive, and those which must as yet remain 

 Experimental. The difference consists in our having 

 been able, or not yet able, to discover marks of marks. 

 If by our various inductions we have been able to pro- 

 ceed no further than to such propositions as these, a a 

 mark of 5, or a and b marks of one another, c a mark 

 of d, or c and d marks of one another, without any- 

 thing to connect a or b with c or d ; we have a science 

 of detached and mutually independent generalizations, 

 such as these, that acids redden vegetable blues, and 

 that alkalis colour them green ; from neither of which 

 propositions could we, directly or indirectly, infer the 

 other : and a science, so far as it is composed of such 

 propositions, is purely experimental. Chemistry, in 

 the present state of our knowledge, has not yet thrown 

 off this character. There are other sciences, however, 

 of which the propositions are of this kind : a a mark 

 of 6, b a mark of c, c of d, d of e, &c. In these 

 sciences we can mount the ladder from a to e by a 

 process of ratiocination ; we can conclude that a is a 

 mark of e, and that every object which has the mark 

 a has the property e, although, perhaps, we never 

 VOL. i. u 



