396 OPERATIONS SUBSIDIARY TO INDUCTION. 



ception, may be owing either to our having no accurate knowledge of 

 the objects themselves, or merely to our not having carefully compared 

 them. Thus a person may have no clear idea of a ship because he has 

 never seen one, or because he remembers but little, and that faintly, 

 of what he has seen. Or he may have a perfect knowledge and 

 remembrance of many ships of various kinds, frigates among the rest, 

 but he may have no clear but only a confused idea of a frigate, because 

 he has not compared them sufficiently to have remarked and remem- 

 bered in what particular points a frigate differs from some other kind 

 of ship. 



It is not, however, necessary, in order to have clear ideas, that we 

 should know all the common properties of the things which we class 

 together. That would be to have our conceptions of the class com- 

 plete as well as clear. It is sufficient if we never class things together 

 without knowing exactly why we do so — without having ascertained 

 exactly what agi'eements we are about to include in our conception ; 

 and if, after having thus fixed our conception, we never vary from it, 

 never include in the class anything which has not those common 

 properties, nor exclude from it anything which has. A clear concep- 

 tion means a determinate conception; one which does not fluctuate, 

 which is not one thing to-day and another to-morrow, but remains fixed 

 and invariable, except when, from the progress of our knowledge, or 

 the correction of some error, we consciously add to it or alter it. 

 A person of clear ideas, is a person who always knows in virtue of 

 what properties his classes are constituted ; what attributes are con- 

 noted by his general names. 



The principal requisites, therefore, of clear conceptions, are habits 

 of attentive observation, an extensive experience, and a memory which 

 receives and retains an exact image of what is obsei-ved. And in 

 proportion as any one has the habit of obsei'ving minutely and com- 

 paring carefiilly a particular class of phenomena, and an accurate 

 memory for the results of the observation and comparison, so will his 

 conceptions of that class of phenomena be clear ; provided he has the 

 indispensable habit, (naturally, however, resulting from those other 

 endowments,) of never using general names without a precise con- 

 notation. 



As the clearness of our conceptions chiefly depends upon the care- 

 fulness and accuracy of our observing and comparing faculties, so their 

 appropriateness, or rather the chance we have of hitting upon the 

 appropriate conception in any case, mainly depends upon the activity 

 of the same faculties. He who by habit, grounded on sufficient naturaJ 

 aptitude, has acquired a readiness in accurately obsei"ving and com- 

 paring phenomena, will perceive so many more agreements and will 

 perceive them so much more rapidly than other people, that the chances 

 are much greater of his perceiving, in any instance, the agreement on 

 which the important consequences depend. 



§ 6. We are not, at the same time, to forget, that the agreement 

 cannot always be discovered by mere comparison of the very phenom- 

 ena in question, without the aid of a conception acquired elsewhere ; 

 as in the case, so often referred to, of the planetary orbits. 



The search for the agreement of a set of phenomena is in truth 

 very similar to the search for a lost or hidden object. At first we placQ 



