Comment by voidhorse
This depends on the assumption that all brain activity is the process of realizing computable functions. I'm not really aware of any strong philosophical or neurological positions that has established this beyond dispute. Not to resurrect vitalism or something but we'd first need to establish that biological systems are reducible to strictly physical systems. Even so, I think there's some reason to think that the highly complex social historical process of human development might complicate things a bit more than just brute force "simulate enough neurons". Worse, whose brain exactly do you simulate? We are all different. How do we determine which minute differences in neural architecture matter?
> we'd first need to establish that biological systems are reducible to strictly physical systems.
Or even more fundamentally, that physics captures all physical phenomena, which it doesn't. The methods of physics intentionally ignore certain aspects of reality and focus on quantifiable and structural aspects while also drawing on layers of abstractions where it is easy to mistakenly attribute features of these abstractions to reality.