166 NAMES AND PROPOSITIONS, 



formed, such as the class animal or plant, or the 

 class sulphur or phosphorus, or the class white or red, 

 and consider in what particulars the individuals in- 

 cluded in the class differ from those which do not 

 come within it, we find a very remarkable diversity in 

 this respect between some classes and others. There 

 are some classes, the things contained in which differ 

 from other things only in certain particulars which 

 may be numbered; while others differ in more than 

 can be numbered, more even than we need ever expect 

 to know. Some classes have little or nothing in com- 

 mon to characterise them by, except precisely what is 

 connoted by the name : white things, for example, are 

 not distinguished by any common properties except 

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

 some way dependent upon, or connected with, white- 

 ness. 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 observations 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 investigation 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 believe 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, therefore, that the properties, on which we 

 ground our classes, sometimes 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 



