Again I'll quote [Wikipedia](https://en.wikipedia.org/wiki/Turing_completeness)
```boxed1
In computability theory, a system of data-manipulation rules (such as a model of computation, a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be used to simulate any Turing machine.
```
This is important because for any Turing-complete system it is impossible to solve
its Halting Problem.

In pondering the nature of Mind, I've come to believe something similar is going
on with Mind and brain. The rough idea is that given total knowledge of the space
outside a person's brain, to the limits possible within the laws of physics,
the long-term behaviour of the brain cannot be predicted. This is, in turn,
related to the long-term unpredictability of a chaotic system, where a tiny
perturbation now will eventually cause the behaviour of the system to diverge
completely from what it would have been without that perturbation. In the case
of the brain, [one neuron can make a difference](/brain/OneNeuronCanMakeADifference),
based on whether it fires or not, in a way that one individual muscle is unlikely
to, and where one extra molecule of a drug is unlikely to.