Biological naturalists (e.g., Godfrey-Smith, Block, Searle, Seth) suggest that computers aren't made of the right kind of stuff to be conscious. Consciousness, they suggest, requires a biological substrate that computers lack. It's not always clear, however, exactly what property animal biologies have that computers lack or why that property matters. It helps, I think, to sort biological naturalism into two flavors. We can then consider what motivates each flavor and see why neither is entirely compelling.
Two Flavors of Biological Naturalism
Flavor One: Computers (at least those built along broadly familiar lines) cannot achieve some crucial type or degree of broad-brush functional or behavioral sophistication required for consciousness. Something -- such as having a metabolism, or being self-organizing in the right way, or having the right kinds of quantum configuration -- is both absent from foreseeable computer architectures and required for achieving some essential broad-brush functional or behavioral organization.
By "broad-brush" I mean functional or behavioral features that are either readily observable from outside or constituted by coarse-grained cognitive mechanisms, such as having a long-term memory store, the capacity to store lexical items and recombine them flexibly in grammatical structures, or the ability extract an object's boundaries from retinal/camera inputs.
Flavor Two: Even if computers can achieve the broad-brush functional and behavioral sophistication of conscious animals, the stuff they're made of still can't support consciousness. Imagine an entity that behaves like a conscious human or dog or frog and has similar broad-brush functional capacities -- close enough that we wouldn't deny consciousness on such broad-brush behavioral or functional grounds. From the outside, it seems conscious, but that appearance is an illusion: Consciousness requires some low-level, fine-grained processes that the system necessarily lacks.
One might read Searle as Flavor Two, Godfrey-Smith as Flavor One, and Block as exploring both flavors -- though there's often some ambiguity.
The Challenge for Flavor One
Flavor One biological naturalism can't, I think, be entirely ruled out. Maybe there's something special about, say, micro-level metabolism or the quantum properties of neural microtubules -- something that enables functionality or behavioral sophistication that will never be practically achievable in standard architecture computers. But this is speculative. Standard computers can do a lot! Even though their processing is digital, classical, and mostly serial, they operate so quickly, and can be so massively linked, that they achieve good approximations of analog and parallel processes. Quantum computers can do some things that ordinary serial computers can't efficiently do, such as quickly factoring large numbers, but ordinary conscious humans can't efficiently do those things either.
To make Flavor One more than a gestural "what if", the biological naturalist must establish two claims. First, they need to argue that some functional or behavioral X is necessary for consciousness. Second, they need to argue that ordinary computers could not realistically achieve X. This will be a challenge! Both claims require some heavy lifting.
We don't know what X's are necessary for consciousness and probably will not know soon. But most leading scientific candidates for X look like just the sorts of things that classical computers could in principle achieve -- having a global workspace, having higher-order self-monitoring, and integrating large amounts of information.
Suppose frogs are conscious (pick your own animal, but select one near the low end of sophistication, compatible with consciousness). Flavor One biological naturalism requires holding that ordinary computers could never match a frog's behavioral and broad-brush functional sophistication. It's true that real-time embodied behavior in a complex world is not something computers are particularly good at. Computers aren't there yet. But the trajectory suggests they might get there, absent some in-principle argument to think they couldn't.
The Challenge for Flavor Two
Flavor Two biological naturalism also can't, I think, be entirely ruled out. Even if we could create the computerized broad-brush behavioral and functional equivalent of a frog, it might not be conscious. Maybe carbon is the right stuff for consciousness and silicon isn't. Or maybe massive parallelism is necessary for consciousness and fast, serial processing, even if it achieves the same computational result, simply won't do.
The challenge for Flavor Two comes from two directions.
The first is thought experiments involving space aliens. As Jeremy Pober and I have recently discussed, behaviorally sophisticated aliens have plausibly evolved in many different substrates in the observable portion of the universe. It would violate Copernican mediocrity to suppose that somehow only we, or only we and a small subset of others, are conscious, while the rest -- just as behaviorally and functionally sophisticated -- lack consciousness. Alien cases suggest we shouldn't insist that an entity must act and function exactly like us or share exactly our substrate down to the particular amino acids and nucleic acids, to be conscious.
Now maybe a computer-chip architecture is just too different -- but why? Once we grant some substrate flexibility on Copernican grounds, the burden of proof shifts to anyone who wants to say such-and-such differences in substrate are consistent with consciousness but such-and-such other differences are not.
The second challenge arises from the de-psychologization of consciousness that Flavor Two requires. Consciousness, one might have thought, would be an important psychological property, playing some important cognitive role. (Which role(s) remains an open question!) But Flavor Two biological naturalism requires denying this plausible psychologism. Otherwise, it collapses into Flavor One.
Picture two systems that don't differ in any of the important psychological (behavioral / functional) properties that we ordinarily associate with consciousness. Both can flexibly learn. Both engage in what seems to be sophisticated language and self-representation. Both coordinate and cooperate with others of their kind in highly sophisicated ways. Both trade short gains against intricate long-term goals. And so on. Yet one is conscious and the other is entirely devoid of consciousness? Although this is possible -- I don't rule it out -- it doesn't seem the most natural conclusion. It needs argument. The burden of proof should land squarely on the biological naturalist who asserts it.
(Is Searle's Chinese room just such an argument? See my reply in Chapter Six of AI and Consciousness -- readable without the first five chapters.)
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