376 THE PRINCIPLES OF SCIENCE. 



itself of all names and other vestiges of antiquity which 

 are not actually useful at the present day. 



Among those ancient expressions which may well be 

 excepted from such considerations and ever retained in 

 use, are the ' Five Words' or ' Five Predicables ' which 

 were described by Porphyry in his ' Introduction to Aris- 

 totle's Organum.' Two of them indeed, namely Genus 

 and Species, are the most venerable names in philosophy, 

 having probably been first employed in their present 

 logical meanings by Socrates. In the present day it 

 requires some mental effort, as Mr. Georges Lewes has 

 remarked ,' to see anything important in the invention 

 of notions now so familiar as those of Genus and Species. 

 But in reality the introduction of such terms showed the 

 rise of the first germs of logic and scientific method : 

 it showed that men were beginning to analyse their pro- 

 cesses of thought. 



The Five Predicables are Genus, Species, Difference, 

 Property, and Accident, or in the original Greek 70/09, 

 eiSos, ($ta<popa, 'IStov, a-v/uLf3e(3t]KO9. Of these, Genus may be 

 taken to mean any class of objects which is regarded as 

 broken up into two minor classes, which form Species 

 of it. The Genus is defined by a certain number of 

 qualities or circumstances which belong to all objects 

 included in the class, and which are sufficient to mark 

 out these objects from all others which we do not intend 

 to include. Interpreted as regards intension, then, the 

 Genus is a group of qualities ; interpreted as regards 

 extension, it is a group of objects possessing those 

 qualities. If now another quality be taken into account 

 which is possessed by some of the objects and not by 

 the others, this quality becomes a Difference which divides 

 the Genus into two Species. We may interpret the Species 



' Biographical History of Philosophy/ (1857) v l- l - P- I2 ^- Grote's 

 ' History of Greece,' vol. viii. p. 578. 



