Full Idea
Nothing that is distinctly conceivable implies a contradiction. Whatever we conceive of as existent we can also conceive as non-existent. So there is no being whose non-existence implies a contradiction. So no being's existence is demonstrable.
Gist of Idea
No being's non-existence can imply a contradiction, so its existence cannot be proved a priori
Source
David Hume (Dialogues Concerning Natural Religion [1751], Part 9)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.95
A Reaction
I totally subscribe to this idea, and take claims that nature actually contains contradictions (based on the inevitable quantum mechanics) to be ridiculous. Nature is the embodiment, chief exemplar and prime test of consistency.
Related Idea
Idea 3929 No moral theory is of any use if it doesn't serve the interests of the individual concerned [Hume]