xxx.] CLASSIFICATION. 705 



question before us is, whether abstraction always increases 

 the number of objects included in a class, which amounts to 

 asking whether the law of the inverse relation of logical 

 quantities is always true. The interest of the question 

 partly arises from the fact, that so high a philosophical 

 authority as Mr. Herbert Spencer has denied that gene- 

 ralisation is implied in abstraction, 1 making this doctrine 

 the ground for rejecting previous methods of classifying 

 the sciences, and for forming an ingenious but peculiar 

 method of his own. The question is also a fundamental 

 one of the highest logical importance, and involves subtle 

 difficulties which have made me long hesitate in forming 

 a decisive opinion. 



Let us attempt to answer the question by examination of 

 a few examples. Compare the two classes gun and iron 

 gun. It is certain that there are many guns which are not 

 made of iron, so that abstraction of the circumstance " made 

 of iron " increases the extent of the notion. Next compare 

 gun and metallic gun. All guns made at the present day 

 consist of metal, so that the two notions seem to be co- 

 extensive ; but guns were at first made of pieces of wood 

 bound together like a tub, and as the logical term gun 

 takes no account of time, it must include all guns that 

 have ever existed. Here again extension increases as in- 

 tension decreases. Compare once more " steam-locomotive 

 engine " and " locomotive engine." In the present day, as 

 far as I am aware, all locomotives are worked by steam, so 

 that the omission of that qualification might seem not to 

 widen the term ; but it is quite possible that in some future 

 age a different motive power may be used in locomotives ; 

 and as there is no limitation of time in the use of logical 

 terms, we must certainly assume that there is a class of 

 locomotives not worked by steam, as well as a class that is 

 worked by steam. When the natural class of Euphorbiacese 

 was originally formed, all the plants known to belong to it 

 were devoid of corollas ; it would have seemed therefore 

 that the two classes " Euphorbiacese," and " Euphorbiaceae 

 devoid of Corollas," were of equal extent. Subsequently a 

 number of plants plainly belonging to the same class were 

 found in tropical countries, and they possessed bright 



1 The Classification of the Sciences, &c., 3rd edit. p. 7. Essays ; 

 Scientific, Political, and Speculative, vol. iii. p. 13. 



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