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mattnewport comments on The two insights of materialism - Less Wrong
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So do you think there is a meaningful difference between computationalism and physicalism if the Church-Turing-Deutsch principle is true? If so, what is it?
Basically, physicalism need not be substrate-independent. For instance, it could be that Pearce is right: subjective experience is implemented by a complex quantum state in the brain, and our qualia, intentionality and other features of subjective experience are directly mapped to the states of this quantum system. This would account for the illusion that our consciousness is "just" our brain, while dramatically simplifying the underlying ontology.
Is that a yes or a no? It seems to me that saying physicalism is not substrate-independent is equivalent to saying the Church-Turing-Deutsch principle is false. In other words, that a Turing machine cannot simulate every physical process. My question is whether you think there is a meaningful difference between physicalism and computationalism if the Church-Turing-Deutsch principle is true. There is obviously a difference if it is false.
Why would this be? Because of free will? Even if free will exists, just replace the input of free will with a randomness oracle and your Turing machine will still be simulating a conscious system, albeit perhaps a weird one.
I don't think free will is particularly relevant to the question. Pearce seems to be claiming that some kind of quantum effects in the brain are essential to consciousness and that a simulation of a brain in a computer therefore cannot be conscious. If you could simulate the quantum processes then the argument falls apart. It only makes sense if the Church-Turing-Deutsch principle is false and there are physical processes that cannot be simulated by a Turing machine. I think that is unlikely but possible and a coherent position.
If all physical processes can be simulated by a Turing machine then I don't see a meaningful difference between physicalism and computationalism. I still don't know what your answer is to that question. If you do think there is still a meaningful difference then please share.
*sigh* You seem to be so committed to computationalism that you're unable to understand competing theories.
Simulating quantum processes on a classical computer is not the same as instantiating them in the real world. And physicalism commits us to giving a special status to the real world, since it's what our consciousness is made of. (Perhaps other "consciousnesses" exist which are made out of something else entirely, but physicalism is silent on this issue.) Hence, consciousness is not invariant under simulation; a classical simulation of a conscious system is similar to a zombie in that it behaves like a conscious being but has no subjective experience.
ETA: I think you are under the mistaken impression that a theory of consciousness needs to explain your heterophenomenological intuitions, i.e. what kinds of beings your brain would model as conscious. These intuitions are a result of evolution, and they must necessarily have a functionalist character, since your models of other beings have no input other than the general form of said beings and their behavior. Philosophy of mind mostly seeks to explain subjective experience, which is just something entirely different.
So you do think there is a difference between physicalism and computationalism even if the Church-Turing-Deutsch principle is true? And this difference is something to do with a special status held by the real world vs. simulations of the real world? I'm trying to understand what these competing theories are but there seems to be a communication problem that means you are failing to convey them to me.
That's what it means to say that physicalism is substrate-dependent. There is a (simple) psycho-physical law which states that subjective experience is implemented on a specific substrate.
It just so happens that evolution has invented some analog supercomputers called "brains" and optimized them for computational efficiency. At some point, it hit on a "trick" for running quantum computations with larger and larger state spaces, and started implementing useful algorithms such as reinforcement learning, aversive learning, perception, cognition etc. on this substrate. As it turns out, the most efficient physical implementations of such quantum algorithms have subjective experience as a side effect, or perhaps as a crucial building block. So subjective awareness got selected for and persisted in the population to this day.
It seems a fairly simple story to me. What's wrong with it?
So is one of the properties of that specific substrate (the physical world) that it cannot be simulated by a Turing machine? I don't know why you can't just give a yes/no answer to that question. I've stated it explicitly enough times now that you just come across as deliberately obtuse by not answering it.
I think I've been fairly clear that I don't deny the possibility that consciousness depends on non-computable physics. I don't think it is the most likely explanation but it doesn't seem to be clearly ruled out given our current understanding of the universe. Your story might be something close to the truth if the Church-Turing-Deutsch principle is false. It appears to me to be incoherent if it is true however.
I think the Church-Turing-Deutsch principle is probably true but I don't think we can rule out the possibility that it is false. If it is true then it seems a simulation of a human running on a conventional computer would be just as conscious as a real human. If it is false then it is not possible to simulate a human being on a conventional computer and it therefore doesn't make sense to say that such a simulation cannot be conscious because a simulation cannot be created. What if anything do you disagree with from those claims?
Because it implies the possibility of zombies, or for some other reason?
Are you saying that there is some extra law (on top of the physical laws that explain how our brains implement our cognitive algorithms) that maps our cognitive algorithms, or a certain way of implementing them, to consiousness? So that, in principal, the universe could have not had that law, and we would do all the same things, run all the same cognitive algorithms, but not be consious? Do you believe that p-zombies are conceptially possible?
The psycho-physical law is not really an extra law "on top of the laws of physics", so much as a correspondence between quantum state spaces and subjective experiences--ideally, the correspondence would be as simple as possible.
You could build a version of the universe which was not endowed with any psycho-physical laws, but it's not something anyone would ever experience; it would be one formal system plucked out seemingly at random from the set of computational structures. It is as logically possible as anything else, but whether it makes sense to regard such a bizarre thing as "conceptually possible" is another matter.