In David Hume's Dialogues Concerning Natural Religion, the character Cleanthes argues that no being could ever be proven to exist through an a priori demonstration:
[T]here is an evident absurdity in pretending to demonstrate a matter of fact, or to prove it by any arguments a priori. Nothing is demonstrable, unless the contrary implies a contradiction. Nothing, that is distinctly conceivable, implies a contradiction. Whatever we conceive as existent, we can also conceive as non-existent. There is no being, therefore, whose non-existence implies a contradiction. Consequently there is no being, whose existence is demonstrable.
There have been many other arguments against ontological proofs such as: Existence precedes essence; Gaunilo's island; Necessary nonexistence; Existence is not a predicate; and Problem of incoherence.
yup, welcome to solipsism Bud
gotta go, whistle blew and all
The day you give your heart to Jesus, He will set you free.