CLASSIFICATION AND THE PREDICABLES. 135 



precisely what is connoted by the name : white things, for ex- 

 ample, are not distinguished by any common properties, except 

 whiteness ; or if they are, it is only by such as are in some way 

 dependent on, or connected with, whiteness. But a hundred 

 generations have not exhausted the common properties of 

 animals or of plants, of sulphur or of phosphorus ; nor do we 

 suppose them to be exhaustible, but proceed to new obser- 

 vations and experiments, in the full confidence of discovering 

 new properties which were by no means implied in those we 

 previously knew. While, if any one were to propose for in- 

 vestigation the common properties of all things which are of 

 the same colour, the same shape, or the same specific gravity, 

 the absurdity would be palpable. We have no ground to be- 

 lieve that any such common properties exist, except such as 

 may be shown to be involved in the supposition itself, or to be 

 derivable from it by some law of causation. It appears, there- 

 fore, that the properties, on which we ground our classes, some- 

 times exhaust all that the class has in common, or contain it 

 all by some mode of implication ; but in other instances we 

 make a selection of a few properties from among not only a 

 greater number, but a number inexhaustible by us, and to 

 which as we know no bounds, they may, so far as we are con- 

 cerned, be regarded as infinite. 



There is no impropriety in saying that, of these two classi- 

 fications, the one answers to a much more radical distinction 

 in the things themselves, than the other does. And if any one 

 even chooses to say that the one classification is made by 

 nature, the other by us for our convenience, he will be right ; 

 provided he means no more than this : Where a certain 

 apparent difference between things (though perhaps in itself of 

 little moment) answers to we know not what number of other 

 differences, pervading not only their known properties, but 

 properties yet undiscovered, it is not optional but imperative 

 to recognise this difference as the foundation of a specific dis 

 tinction ; while, on the contrary, differences that are merely 

 finite and determinate, like those designated by the words 

 white, black, or red, may be disregarded if the purpose for 

 which the classification is made does not require attention 



