Indeed a classic (upvoted!), but it presupposes that there is a theory of everything that can be used to perfectly simulate the universe, and I'd like to remind everybody that we still don't know whether such a thing exists. Nor do we know whether it's possible to simulate "just" neurons and get meaningful results on some higher level. People have been trying at that for decades, so far without success. (nice overview here: https://ccli.substack.com/p/the-biggest-mystery-in-neuroscie...)

Our bodies could just be rocks on a beach, but whatever it is about us that is capable of observing the output of our own internal thinking (preventing us from just being p-zombies) is not contained within the beach.