It's definitely not arbitrary. We have solid mathematical definitions of computing that are not tied to any particular implementation. Saying "the brain is not

It's definitely not arbitrary. We have solid mathematical definitions of computing that are not tied to any particular implementation. Saying "the brain is not a computer" is a shorthand for "the brain is not Turing equivalent". Now, that could be true. But if it is, it would be the first time anybody has found a physical system in our universe that can't be simulated by a Turing machine. It seems like quite a stretch.