being = self-attributed existence
To merely sentient beings (animals, roughly), there is presumably no notion of self-identity. The concept of being can only take root in a cognizant mind, and given the p-xombie gap, can only be evaluated from within. All of which is to say, it's tautological. I am because i say i am. No further evidence is necessary or indeed possible.
This provides a foundation for understanding everything else in the universe in explicit detail.