Bibio, ergo sum.

I drink therefore I am.

Proof: Bobert has a beer and the beer stays inside him for a period.
If he did not exist it would all slosh out on the ground/floor as it was poured out. It does not do this, so something called "Bobert" encloses it. The container could be as easily called "Dipshit" or "Sadie" or "Model XM-71a" but in this case it is self-named "Bobert." As long as the poured beer remains beer and does not slosh around on the floor Bobert exists.