(1) A computer is anything that does computations, and a computation is just a set of states that are partially ordered with respect to a “next” state relation.

(1) A computer is anything that does computations, and a computation is just a set of states that are partially ordered with respect to a “next” state relation. (2) A universal computer is a computer that can simulate any other computer with some bounded slowdown in performance (the actual important thing about such computers is that they can simulate better computers). So, while the brain seems to have a universal computer (since it can design and run universal computers), it’s not obvious that a universal computer can simulate a brain without infinite slowdown. Some evidence against brains being equivalent to Universal Turing Machines is that some brains can perceive real numbers. Thus, it’s conceivable that even if the mind (the thing that is realized by a brain) is a partially ordered set of states, it’s possible that the states themselves are not computable. Now, I am not claiming that we cannot eventually build a brain, just suggesting that maybe the only thing that can simulate a brain is another brain.