A person of clear ideas is a person who always knows in virtue of what properties his classes are constituted
 John Stuart Mill, A System Of Logic, Ratiocinative And Inductive (1843). copy citation

Context

“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. 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 observed.” source