Full Idea
Whatever 'is' may 'not be'. No negation of a fact can involve a contradiction. The non-existence of any being, without exception, is as clear and distinct an idea as its existence.
Gist of Idea
It can never be a logical contradiction to assert the non-existence of something thought to exist
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