I think Rap's proof is one of the best. As the person most involved in cleaning floors at my own house, I can see the compelling argument that if Bobert did not exist, the beer he drinks would mess up the floor.

But on a higher philosophic level we know Bobert exists because he mistyped 'proof' as 'proff.'   If Bobert were some kind of hologram generated by evil AI, with language supplied by downloaded lexicons, he couldn't misspell anything even if he wanted to.

"I err, therefore I'm human."