206 OPERATIONS SUBSIDIARY TO INDUCTION. 



out clear ideas, is one who habitually classes together, under 

 the same general names, things which have no common pro- 

 perties, or none which are not possessed also by other things ; 

 or who, if the usage of other people prevents him from actually 

 misclassing things, is unable to state to himself the common 

 properties in virtue of which he classes them rightly. 



But it is not the sole requisite of classification that the 

 classes should be real classes, framed by a legitimate mental 

 process. Some modes of classing things are more valuable 

 than others for human uses, whether of speculation or of 

 practice ; and our classifications are not well made, unless the 

 things which they bring together not only agree with each 

 other in something which distinguishes them from all other 

 things, but agree with each other and differ from other 

 things in the very circumstances which are of primary im- 

 portance for the purpose (theoretical or practical) which we 

 have in view, and which constitutes the problem before us. 

 In other words, our conceptions, though they may be clear, 

 are not appropriate for our purpose, unless the properties we 

 comprise in them are those which will help us towards what 

 we wish to understand i. e., either those which go deepest 

 into the nature of the things, if our object be to understand 

 that, or those which are most closely connected with the par- 

 ticular property which we are endeavouring to investigate. 



We cannot, therefore, frame good general conceptions 

 beforehand. That the conception we have obtained is the one 

 we want, can only be known when we have done the work for 

 the sake of which we wanted it ; when we completely under- 

 stand the general character of the phenomena, or the conditions 

 of the particular property with which we concern ourselves. 

 General conceptions formed without this thorough knowledge, 

 are Bacon's " notiones temere a rebus abstracts. " Yet such 

 premature conceptions we must be continually making up, in 

 our progress to something better. They are an impediment 

 to the progress of knowledge, only when they are permanently 

 acquiesced in. When it has become our habit to group things 

 in wrong classes in groups which either are not really classes, 

 having no distinctive points of agreement (absence of clear 



