Full Idea
If I ask why you believe some fact, you must tell me a reason, which will be some other fact, connected with it. But this process must terminate in a fact which is present to your memory or senses; or you must allow that the belief is without foundation.
Gist of Idea
Reasons for belief must eventually terminate in experience, or they are without foundation
Source
David Hume (Enquiry Conc Human Understanding [1748], V.I.37)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.46
A Reaction
A classic quotation of empirical foundationalism. The rival view would be that the process does not terminate at all, but nevertheless builds up a persuasive picture which is foundational.