Consciousness is not like Photosynthesis

I've been posting some comments on Massimo Pigliucci's blog recently in defense of Strong AI, after a couple of posts attacking the computational theory of mind.

In the first, he uses John Searle's Chinese Room argument to demonstrate that a computer cannot be conscious. If you've been paying attention to this blog, you should know what I think of that argument!

In the second post, he uses an article by Jack Copeland which undermines the position of philosophers who take the Church-Turing thesis to imply that a Turing machine can do any computation performable by any machine. Unfortunately, Pigliucci seems to fail to realise that while Copeland is technically correct, the philosophers he criticises are no less so unless some very unlikely propositions turn out to be true (i.e. that the laws of physics are not computable and the brain is a hypercomputer).

But what motivated me to write this post is the same tired old argument that keeps coming up again and again in these discussions, the analogy to photosynthesis.

Pigliucci is a biological naturalist, which means that he thinks that although the brain is essentially just a machine that obeys the laws of physics like any other ball of matter, there's something unknown which is intrinsic to the biology of brains which gives rise to consciousness. In other words, you probably can't have consciousness without biology, and certainly not until we understand that biology well enough to physically mimic it. In other words, computation is not enough.

He likes to explain this position and why he doubts the computational theory of mind by analogy to photosynthesis. This analogy came up repeatedly in a discussion with the wonderful Eliezer Yudkowsky on bloggingheads.tv, despite Yudkowsky's efforts to explain why it doesn't work (efforts which in my view succeed entirely).

Massimo Pigliucci: ... photosynthesis [...] is very well understood at a chemical level [... and at ...] the logical processing level. 

[...]

You can in fact simulate those chemical reactions inside a computer. [...] The fact is of course that you can simulate all you want, the one thing you're not going to get out of the simulation is sugar, which is the outcome of photosynthesis. And the reason for that is because what's important about photosynthesis is not just the logical structure of the process, it's the actual physical particular implementation. Unless you have certain kinds of chemicals that work in a certain way [...] you just don't have photosynthesis.

 - Edited from video discussion transcript

This harks back to arguments by John Searle and others making joking statements to the effect that we don't need to call the fire department if a simulated fire has got out of control, or that a simulated hurricane isn't likely to flood your apartment.

These arguments are all the same and they are all very poor.

The reason is simple. The brain is an information processing machine, and so the only product the brain needs to make reliably is output. Output is not a physical product, it is information. Like a novel, which can be paperback or hardback. audio book or ebook, as long as the content is the same then nothing essential has changed.

When I pointed this out to Massimo, he responded with astonishment as though I claimed that the brain did not produce any physical chemicals at all.

Of course this is not my position. I am well aware that the brain produces many hormones, neurotransmitters and other chemicals that have an impact on the brain and indeed the body at large. However, as long as we keep in mind the self-evident proposition that the job of the brain as an information processing device is to process information, the chemicals are not important as ends in themselves (unless consciousness is a chemical!). If the brain were replaced by a perfectly equivalent electronic device, these chemicals would not be produced but the information processing would be unaffected. Whatever functions the chemicals had once implemented would have been replaced by an electronic analogue.

In photosynthesis, the product is sugar -- a physical substance. Of course you can't get physical real-world sugar from a simulation of photosynthesis. The best you can hope for is virtual sugar.

So why don't we say that the best we could hope for from a simulated brain is virtual output? It should be clear that that doesn't really make sense. Unlike virtual sugar, virtual output is pretty much the same thing as real output, and can be put to real world use. Information cannot be virtual because it is not physical in the first place.

But output is not Massimo's concern. He is concerned with consciousness. Perhaps he thinks that the brain's job is to produce consciousness, or perhaps consciousness is just a necessary part of its information processing job. In either case, unless consciousness is a physical substance or a state of physical matter, (notions as outmoded as the vitalist assumption that a substance called élan vital is responsible for life), he and his colleagues have no business making analogies to photosynthesis, hurricanes or fire.

According to the computational theory of mind (or strong AI as I have been referring to it in previous posts), consciousness is not a physical substance but an aspect of certain sophisticated ongoing computations. If this is correct, then consciousness achieved in a simulation is every bit as real as that achieved in a physical brain, for the same reason that a simulated calculator is still a calculator.

Quite simply, there is no such thing as a simulated computation. Once you're simulating a computation, you're doing the computation.

So please, please, please stop making analogies to physical processes when there is every reason to believe that consciousness is not a physical phenomenon.