Full Idea
A cause is an object, followed by another, and where all the objects similar to the first are followed by objects similar to the second, or, an object followed by another, and whose appearance always conveys the thought to that other.
Gist of Idea
A cause is either similar events following one another, or an experience always suggesting a second experience
Source
David Hume (Enquiry Conc Human Understanding [1748], VII.II.60)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.76