Full Idea
Without exception, knowledge of cause and effect is not attained by reasonings a priori, but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other.
Gist of Idea
No causes can be known a priori, but only from experience of constant conjunctions
Source
David Hume (Enquiry Conc Human Understanding [1748], IV.I.23)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.27