144 NAMES AND PROPOSITIONS. 



nothing ; it merely denotes the attribute corresponding to a 

 certain sensation : but if we are making a classification of 

 colours, and desire to justify, or even merely to point out, the 

 particular place assigned to whiteness in our arrangement, we 

 may define it &quot; the colour produced by the mixture of all the 

 simple rays ;&quot; and this fact, though by no means implied in 

 the meaning of the word whiteness as ordinarily used, but 

 only known by subsequent scientific investigation, is part of 

 its meaning in the particular essay or treatise, and becomes 

 the differentia of the species.* 



The differentia, therefore, of a species may be defined 

 to be, that part of the connotation of the specific name, 

 whether ordinary or special and technical, which distin 

 guishes the species in question from all other species of the 

 genus to which on the particular occasion we are refer 

 ring it. 



7. Having disposed of Genus, Species, and Differentia, 

 we shall not find much difficulty in attaining a clear con 

 ception of the distinction between the other two predicates, 

 as well as between them and the first three. 



In the Aristotelian phraseology, Genus and Differentia 

 are of the essence of the subject; by which, as we have seen, 

 is really meant that the properties signified by the genus 

 and those signified by the differentia, form part of the con 

 notation of the name denoting the species. Pjojrium and 

 Accidens, on the other hand, form no part of the essence, but 

 are predicated of the species only accidentally. Both are 

 Accidents, in the wider sense in which the accidents of a 

 thing are opposed to its essence ; though, in the doctrine of 

 the Predicables, Accidens is used for one sort of accident 

 I only, Proprium being another sort. Proprium, continue the 

 schoolmen, is predicated accidentally, indeed, but necessarily ; 



* If we allow a differentia to what is not really a species. For the distinc 

 tion of Kinds, in the sense explained by us, not being in any way applicable to 

 attributes, it of course follows that although attributes may be put into classes, 

 those classes can be admitted to be genera or species only by courtesy. 



