Full Idea
Relations of Ideas are discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe.
Gist of Idea
Relations of ideas are known by thought, independently from the world
Source
David Hume (Enquiry Conc Human Understanding [1748], IV.I.20)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.25