Full Idea
A thing necessarily exists if no cause or reason be granted which prevents its existence. No cause can be given which prevents the existence of God, or which destroys his existence, so we must conclude that he necessarily exists.
Gist of Idea
God must necessarily exist, because no reason can be given for his non-existence
Source
Baruch de Spinoza (The Ethics [1675], I Pr 11)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.52
A Reaction
I can't think of any reason why there shouldn't be a giant rat which fills a large proportion of the universe. Indeed, it may be the missing 'dark matter'. So presumably it has necessary existence. Proving non-existence is obviously tricky.