[bbor]

What’s not exact about a human brain…? Complex, sure. But I don’t see the difference in kind you allude to

[navaati]

It does not help you having a perfect and exact rule from initial conditions to future prediction... if you cannot have a complete and exact description of your initial conditions.

We might never be able to get this exact description of the internal state of a brain, this is what makes perfect prediction impossible.

[burnished]

As it turns out, pretty much everything. Individual neurons are imprecise and adaptive and they form networks that show chaotic behavior.

Basically I'd turn the question around - what makes you think brains are exact?