Full Idea
When we entertain any suspicion that a philosophical term is without any meaning or idea, we need but enquire "from what impression is that supposed idea derived?"
Gist of Idea
If we suspect that a philosophical term is meaningless, we should ask what impression it derives from
Source
David Hume (Enquiry Conc Human Understanding [1748], II.17)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.22