Full Idea
The principle requisites 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.
Gist of Idea
Clear concepts result from good observation, extensive experience, and accurate memory
Source
John Stuart Mill (System of Logic [1843], 4.2.5)
Book Reference
Mill,John Stuart: 'System of Logic (9th ed, 2 vols)' [Longmans, Green etc 1875], p.206
A Reaction
Empiricists are always crying out for people to 'attend to the evidence', and this is the deeper reason why. Not only will one know the world better in a direct way, but one will actually think more clearly. Darwin is the perfect model for this.