CLASSIFICATION AND THE PREDICABLES. 141 



to prevent it from denoting individuals not included in the 

 genus. And it must connote something besides, otherwise it 

 would include the whole genus. Animal denotes all the indi- 

 viduals denoted by man, and many more. Man, therefore, 

 must connote all that animal connotes, otherwise there might 

 be men who are not animals ; and it must connote something 

 more than animal connotes, otherwise all animals would be 

 men. This surplus of connotation this which the species 

 connotes over and above the connotation of the genus is the 

 Differentia, or specific difference ; or, to state the same propo- 

 sition in other words, the Differentia is that which must be 

 added to the connotation of the genus, to complete the conno- 

 tation of the species. 



The word man, for instance, exclusively of what it con- 

 notes in common with animal, also connotes rationality, and 

 at least some approximation to that external form which we 

 all know, but which as we have no name for it considered in 

 itself, we are content to call the human. The Differentia, or 

 specific difference, therefore, of man, as referred to the genus 

 animal, is that outward form and the possession of reason. 

 The Aristotelians said, the possession of reason, without the 

 outward form. But if they adhered to this, they would have 

 been obliged to call the Houyhnhnms men. The question 

 never arose, and they were never called upon to decide how 

 such a case would have affected their notion of essentiality. 

 However this may be, they were satisfied with taking such a 

 portion of the differentia as sufficed to distinguish the species 

 from all other existing things, though by so doing they might 

 not exhaust the connotation of the name. 



6. And here, to prevent the notion of differentia from 

 being restricted within too narrow limits, it is necessary to 

 remark, that a species, even as referred to the same genus, 

 will not always have the same differentia, but a different one, 

 according to the principle and purpose which preside over the 

 particular classification. For example, a naturalist surveys 

 the various kinds of animals, and looks out for the classifica- 

 tion of them most in accordance with the order in which, for 



