author: niplav, created: 2024-03-23, modified: 2024-04-27, language: english, status: finished, importance: 3, confidence: unlikely

I don't think that humans are (equivalent to) Turing machines, but people who defend the view are barking up the right tree.

Trying to Disambiguate Different Questions about Whether Humans are Turing Machines

I often hear the sentiment that humans are Turing machines, and that this sets humans apart from other pieces of matter.

Examples are this thread and a short section in an interview with David Deutsch:

So all hardware limitations on us boil down to speed and memory capacity. And both of those can be augmented to the level of any other entit y that is in the universe. Because if somebody builds a computer that can think faster than the brain, then we can use that very computer or t hat very technology to make our thinking go just as fast as that. So that's the hardware. […] So if we take the hardware, we know that our brains are Turing-complete bits of hardware, and therefore can exhibit the functionality of running any computable program and function.


So the more memory and time you give it, the more closely it could simulate the whole universe. But it couldn't ever simulate the whole univ erse or anything near the whole universe because it is hard for it to simulate itself. Also, the sheer size of the universe is large.

I've always found those statements a bit strange and confusing, so it seems worth it to tease apart what they could mean.

The question "is a human a Turing machine" is probably meant to convey "can a human mind execute arbitrary programs?", that is "are the languages the human brain emit at least recursively enumerable?", as opposed to e.g. context-free languages.

  1. My first reaction is that humans are definitely not Turing machines, because we lack the infinite amount of memory the Turing machine has in form of an (idealized) tape. Indeed, in the Chomsky hierarchy human aren't even at the level of pushdown automata (since we lack an infinitely deep stack), instead we are nothing more than finite state automata. (I remember a professor pointing this out to us that all physical instantiations of computers are merely finite-state automata).
    1. Depending on one's interpretation of quantum mechanics, one might instead argue that we're at least nondeterministic finite automata or even Markov chains. However, every nondeterministic finite automaton can be transformed into a deterministic finite automaton, albeit at an exponential increase in the number of states, and Markov chains aren't more computationally powerful (e.g. they can't recognize Dyck languages, just as DFAs can't).
    2. It might be that Quantum finite automata are of interest, but I don't know enough about quantum physics to make a judgment call.
  2. The above argument only applies if we regard humans as closed systems with clearly defined inputs and outputs. When probed, many proponents of the statement "humans are Turing machines" indeed fall back to a motte that in principle a human could execute every algorithm, given enough time, pen and paper.
    1. This seems true to me, assuming that the matter in universe does not have a limited amount of computation it can perform.
      1. In a finite universe we are logically isolated from almost all computable strings, which seems pretty relevant.
      2. Another constraint is from computational complexity; should we treat things that are not polynomial-time computable as basically unknowable? Humans certainly can't solve NP-complete problems efficiently.
    2. I'm not sure this is a very useful notion.
      1. On the one hand, I'd argue that, by orchestrating the exactly right circumstances, a tulip could receive specific stimuli to grow in the right directions, knock the correct things over, lift other things up with its roots, create offspring that perform subcomputations &c to execute arbitrary programs. Conway's Game of Life certainly manages to! One might object that this is set up for the tulip to succeed, but we also put the human in a room with unlimited pens and papers.
      2. On the other hand, those circumstances would have to be very exact, much more so than with humans. But that again is a difference in degree, not in kind.


After all, I'm coming down with the following belief: Humans are certainly not Turing machines, however there might be a (much weaker) notion of generality that humans fulfill and other physical systems don't (or don't as much). But this notion of generality is purported to be stronger than the one of life:

$$\text{Turing-completeness} \Rightarrow \text{Context-sensitive} \Rightarrow \text{Context-free} \overset{?}{\Rightarrow} \text{Proposed-generality} \overset{?}{\Rightarrow} \text{Life} \overset{?}{\Rightarrow} \text{Finite-state automata}$$

I don't know of any formulation of such a criterion of generality, but would be interested in seeing it fleshed out.

See Also