226 OPERATIONS SUBSIDIARY TO INDUCTION. 



ness, or, as it may be otherwise called, this vagueness, 

 in the general conception, may be owing either to 

 our having no accurate knowledge of the objects them- 

 selves, or merely to our not having carefully com- 

 pared 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 com- 

 pared 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 pro- 

 perties 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 

 agreements 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 any- 

 thing which has not those common properties, nor 

 exclude from it anything which has. A clear con- 

 ception 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 connoted by his 

 general names. 





