704 THE PRINCIPLES OF SCIENCE. [CHAP. 



of "the matchless beauty of the Eamean Tree." After 

 fully showing its logical value as an exhaustive method of 

 classification, and refuting the objections of Eeid and 

 Kames, on a wrong ground, as I think, he proceeds to 

 inquire to what length it may be carried. He correctly 

 points out two objections to the extensive use of bifid 

 arrangements, (i) that they soon become impracticably 

 extensive and unwieldy, and (2) that they are unecono- 

 mical. In his day the recorded number of different species 

 of plants was 40,000, and he leaves the reader to estimate 

 the immense number of branches and the enormous area of 

 a bifurcate table which should exhibit all these species in 

 one scheme. He also points out the apparent loss of 

 labour in making any large bifurcate classification; but 

 this he considers to be fully recompensed by the logical 

 value of the result, and the logical training acquired in its 

 execution. Jeremy Bentham, then, fully recognises the 

 value of the Logical Alphabet under another name, though 

 he apprehends also the limit to its use placed by the 

 finiteness of our mental and manual powers. 



Does Abstraction imply Generalisation 1 ! 



Before we can acquire a sound comprehension of the 

 subject of classification we must answer the very difficult 

 question whether logical abstraction does or does not imply 

 generalisation. It comes to exactly the same thing if we 

 ask whether a species may be coextensive with its genus, 

 or whether, on the other hand, the genus must contain 

 more than the species. To abstract logically is (p. 27), 

 to overlook or withdraw our notice from some point of 

 difference. Whenever we form a class we abstract, for the 

 time being, the differences of the objects so united in respect 

 of some common quality. If we class together a great 

 number of objecte as dwelling-houses, we overlook the fact 

 that some dwelling-houses are constructed of stone, others 

 of brick, wood, iron, &c. Often at least the abstraction of a 

 circumstance increases the number of objects included 

 under a class according to the law of the inverse relation 

 of the quantities of extension and intension (p. 26). 

 Dwelling-house is a wider term than brick-dwelling-house. 

 House is more general than dwelling-house. But the 



