Full Idea
Only when two species of objects are constantly conjoined can we infer one from the other; were an entirely singular effect presented, which could not be comprehended under a species, I do not see that we could form any conjecture concerning its cause.
Gist of Idea
If a singular effect is studied, its cause can only be inferred from the types of events involved
Source
David Hume (Enquiry Conc Human Understanding [1748], XI.115)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.148
A Reaction
A key issue in causation. Note that Hume is willing to discuss causation in a freakishly unique happening, but only if he can spot a 'type' in the each of the events. I don't like it, but the man has a good point…