Full Idea
If we just reason a priori, anything may appear able to produce anything. The falling of a pebble may, for aught we know, extinguish the sun.
Gist of Idea
A priori it looks as if a cause could have absolutely any effect
Source
David Hume (Enquiry Conc Human Understanding [1748], XII.III.132)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.164