I doubt therefore I am.
I think therefore I am.
I am being deceived therefore I am.
I post therefore I am.
Posting is existing on this forum.
If someone creates an account but never posts anything, one could argue they don't exist, since no one is aware of them.
(Unless, if course, they react to other people's posts, in which case they do exist and users become aware of their presence, and my little philosophical exercise makes no sense and I'd be wise to go vent in The Lounge and stay away from threads that are out of my depth.)