390 THE PRINCIPLES OF SCIENCE. 



partly arises from the fact, that so high a philosophical 

 authority as Mr. Herbert Spencer has denied that gene- 

 ralization is implied in abstraction*, 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, I believe, 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 intension decreases. Compare once more 

 ' steam-locomotive engine' and * locomotive engine.' In 

 the present day so far as I am aware all locomotives are 

 worked by steam, so that the omission of that qualifica- 

 tion 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 origin- 

 ally formed, all the plants known to belong to it were 

 devoid of corollas ; it would have seemed therefore that 

 the two classes ' Euphorbiacese/ and 'Euphorbiacese devoid 

 x ' The Classification of the Sciences,' &c., 3rd edit. p. 7. 



