Full Idea
I may conceive in my own thoughts the idea of a tree, but that is all. And this is far from proving that I can conceive it existing out of the minds of all spirits.
Gist of Idea
I conceive a tree in my mind, but I cannot prove that its existence can be conceived outside a mind
Source
George Berkeley (Three Dialogues of Hylas and Philonous [1713], I p.184)
Book Reference
Berkeley,George: 'The Principles of Human Knowledge etc.', ed/tr. Warnock,G.J. [Fontana 1962], p.184
A Reaction
If Berkeley has based a world view on this point, then his mistake is to require a 'proof'. Aristotle explained why you can't prove everything (not to mention Gödel).