Mindless Intelligence and Smart Sentience
Magnus Vinding interviews David Pearce about
digital (in)sentience and the binding problem

MV: Whether digital sentience is possible would seem to matter greatly for our priorities, and so gaining even slightly more refined views on this matter could be quite valuable. Many people appear to treat the possibility, if not indeed the imminence, of digital sentience as a foregone conclusion. David Pearce, in contrast, is skeptical.

Pearce has written and spoken elaborately about his views on consciousness. My sense, however, is that these expositions do not always manage to clearly convey the core, and actually very simple reasons underlying Pearce’s skepticism of digital sentience. My aim in this interview is to probe Pearce so as to shed greater — or perhaps most of all simpler — light on why he is skeptical, and thus to hopefully advance the discussions on this issue among altruists working to reduce future suffering.

You are skeptical about the possibility of digital sentience. Could you explain why in simple terms?

DP: Sure. Perhaps we can start by asking why so many people believe that our machines will become conscious (cf. How is consciousness defined?). Consciousness is widely recognised to be scientifically unexplained. But the computer metaphor of mind seems to offer us clues (cf. Why is the brain like a computer?). As far as I can tell, many if not most believers in digital sentience tend to reason along the following lines. Any well-defined cognitive task that the human mind can perform could also be performed by a programmable digital computer (cf. Turing machine). A classical Turing machine is substrate-neutral. By “substrate-neutral”, we mean that whether a Turing machine is physically constituted of silicon or carbon or gallium oxide (etc) makes no functional difference to the execution of the program it runs. It’s commonly believed that the behaviour of a human brain can, in principle, be emulated on a classical Turing machine. Our conscious minds must be identical with states of the brain. If our minds weren’t identical with brain states, then dualism would be true. Therefore, the behaviour of our minds can in principle be emulated by a digital computer. Moreover, the state-space of all possible minds is immense, embracing not just the consciousness of traditional and enhanced biological lifeforms, but also artificial digital minds and digital superintelligence. Accordingly, the belief that non-biological information-processing machines can’t support consciousness is arbitrary. It’s unjustified carbon chauvinism.

I think most believers in digital sentience would recognise that the above considerations are not a rigorous argument for the existence of inorganic machine consciousness. The existence of machine consciousness hasn’t been derived from first principles. The “explanatory gap” is still unbridged. Yet what is the alternative?

Well, I'm a scientific rationalist. I’m also an unbeliever in non-trivial digital sentience. Digital computers and the software they run are not phenomenally-bound subjects of experience (cf. The Binding Problem). Ascribing sentience to digital computers or silicon robots is a form of anthropomorphic projection – a projection their designers encourage by giving their creations traditional human names (“Watson”, “Sophia”, “Alexa” etc). Moreover, our biological minds can perform feats that defeat any classical digital computer, ranging from the investigation of state-spaces of consciousness (cf. Psychedelic exploration) to exploring the nature of (dis)value disclosed by the pain-pleasure axis. Digital zombies don’t just lack understanding; they will never know why anything matters.

Before defending the claim that digital computers are zombies, I will lay out two background assumptions.
My first background assumption might seem scarcely relevant to your question. Perpetual direct realism is false. Inferential realism about the external world is true. The subjective contents of your consciousness aren’t merely a phenomenally thin and subtle serial stream of logico-linguistic thought-episodes playing out behind your forehead, residual after-images when you close your eyes, inner feelings and emotions and so forth. Consciousness is also your entire phenomenal world-simulation – what naïve realists call the "publicly accessible external world". Unless you have the neurological syndromes of simultanagnosia (the inability to see more than one object at once) or akinetopsia (“inability to see motion”), you can simultaneously experience a host of dynamic objects – for example, 22 players on a football pitch, or an advancing pride of hungry lions. These perceptual objects populate your virtual world of experience from the sky above to your body-image below. Consciousness is all you directly know. The external environment is an inference, not a given.

Let’s for now postpone discussion of how our skull-bound minds are capable of such an extraordinary feat of real-time virtual world-making. The point is that if you couldn’t experience multiple feature-bound phenomenal objects – i.e. if you were just an aggregate of 86 billion membrane-bound neuronal “pixels” of experience or neuronal feature-processors communicating across chemical and electrical synapses – then you’d be helpless. Compare dreamless sleep. Like your enteric nervous system (the “brain-in-the-gut”), your brain would still be a fabulously complex information-processing system. But you’d risk starving to death or getting eaten. The phenomenenal binding of waking consciousness is immensely adaptive (cf. What is the "purpose" of consciousness).

My second assumption is physicalism. I assume the unity of science. All the special sciences (chemistry, molecular biology etc) reduce to physics. In principle, the behaviour of organic macromolecules such as self-replicating DNA can be described entirely in the mathematical language of physics without mentioning “life” at all, though such high-level description is convenient. Complications aside, no element of reality is missing from the mathematical formalism of our best theory of the world, quantum mechanics, or more strictly from tomorrow’s unification of quantum field theory and general relativity. One corollary of physicalism is that only “weak” emergence is permissible. Just as the behaviour of programs running on your PC supervenes on the behaviour of its machine code, likewise the behaviour of biological organisms can in principle be exhaustively reduced to quantum chemistry and thus ultimately to quantum field theory. The conceptual framework of physicalism is traditionally associated with materialism. According to materialism as broadly defined, the intrinsic nature of the physical – more poetically, the mysterious “fire” in the equations – is non-experiential. Indeed, the assumption that quantum field theory describes fields of insentience is normally treated as too trivially obvious to be worth stating. However, this assumption of insentience leads to the Hard Problem of consciousness. But non-materialist physicalism drops this plausible metaphysical assumption. If the intrinsic nature argument is sound, there is no Hard Problem of consciousness: it’s the essence of the physical). However, both “materialist” physicalists and non-materialist physicalists agree: everything that happens in the world is constrained by the mathematical straitjacket of modern physics. Any supposedly emergent phenomenon must be derived, ultimately, from physics. Irreducible “strong” emergence would be akin to magic.

Anyhow, the reason I don’t believe in digital minds is that classical computers are incapable of phenomenal binding. If we make the standard assumption that their 1s and 0s and logic gates are non-experiential, then digital computers are zombies. Less obviously, digital computers are zombies if we don’t make this standard assumption! Imagine, fancifully, replacing non-experiential 1 and 0s of computer software with discrete “pixels” of experience. Run the program as before. The upshot will still be a zombie (though technically a micro-experiential zombie. What’s more, neither increasing the complexity of the code nor exponentially increasing the speed of its execution could cause discrete “pixels” somehow to blend into each other in virtue of their functional role, let alone create phenomenally-bound perceptual objects or a unitary self experiencing a unified phenomenal world. The same is true of a connectionist system (cf. Connectionism (Wikipedia)), supposedly more closely modelled on the brain – however well-connected and well-trained the network, and regardless whether its nodes are experiential or non-experiential. The synchronous firing of distributed feature-processors in a “trained up” connectionist system doesn’t generate a unified perceptual object – again on pain of “strong” emergence. AI programmers and roboticists can use workarounds for the inability of classical computers to bind, but they are just – workarounds.

The believer in digital sentience can protest that we don’t know that phenomenal minds can’t emerge at some level of computational abstraction in digital computers...and the believer is right! If abstract objects have the causal power to create conscious experience, then digital computer programs might be subjects of experience. But recall we’re here assuming physicalism. If physicalism is true, then even if consciousness is fundamental to the world, we can know that digital computers are at most micro-experiential zombies.

Of course, monistic physicalism may be false. “Strong” emergence may be real. But if so, then reality is fundamentally lawless. The scientific world-picture would be false.

Yet how do biological minds routinely accomplish binding if phenomenal binding is impossible for classical digital computers (cf. Universal Turing machine). Even if our neurons support rudimentary “pixels” of experience, why aren’t animals like us in the same boat as classical digital computers and classically parallel connectionist systems?

I can give you my tentative answer. Naïvely, it’s the reductio ad absurdum of quantum mind: “Schrödinger’s neurons”:
Quantum minds. And surprisingly, it’s experimentally falsifiable via interferometry:
Quantum mind (Wikipedia)

Yet the conjecture I explore may conceivably be of interest only to someone who already feels the force of the binding problem. Plenty of researchers would say it’s a ridiculous solution to a nonexistent problem. I agree it’s a crazy solution; but IMO it’s worth experimentally falsifying. Other researchers just lump phenomenal binding together with the Hard Problem (cf. How to categorise the binding problem) as one big insoluble mystery they suppose can be quarantined from the rest of scientific knowledge.
Their defeatism and optimism alike are premature.

MV: Thanks, David. A lot to discuss there, obviously.

Perhaps the most crucial point to really appreciate in order to understand your skepticism is that you are a strict monist about reality. That is, the mind is not something over and above the physical, but rather identical with it (which is really not an intuitive way for us to think). And so if “the mental” and “the physical” are essentially the same ontological thing, or phenomenon, under two different descriptions, then there must, roughly speaking, also be a match in terms of their topological properties.

As Mike Johnson explained your view: “consciousness is ‘ontologically unitary’, and so only a physical property that implies ontological unity ... could physically instantiate consciousness.” (Principia Qualia, p. 73). (Note that “consciousness” here refers to an ordered, composite mind; not phenomenality more generally.)

Conversely, a system that is physically discrete or disconnected — say, a computer composed of billiard balls that bump into each other, or lighthouses that exchange signals across hundreds of kilometers — could not, in your view, support a unitary mind. In terms of the analogy of thinking about consciousness as waves, your view is roughly that we should think of a unitary mind as a large, composite wave of sorts, akin to a song, whereas disconnected “pixels of experience” are like discrete microscopic proto-waves, akin to tiny disjoint blobs of sound. (And elsewhere you quote Seth Lloyd saying something similar about classical versus quantum computations: “A classical computation is like a solo voice – one line of pure tones succeeding each other. A quantum computation is like a symphony – many lines of tones interfering with one another.”)

This is why you say that “computer software with discrete ‘pixels’ of experience … will still be … a micro-experiential zombie”, and why you say that “even if consciousness is fundamental to the world, we can know that digital computers are at most micro-experiential zombies” — it’s because of this physical discreteness, or “disconnectedness”.

And this is where it seems to me that the computational view of mind is also starkly at odds with common sense, as well as with monism. For it seems highly counterintuitive to claim that billiard balls bumping into each other, or lighthouses separated by hundreds of kilometers that exchange discrete signals, could, even in principle, mediate a unitary mind. I wonder whether most people who hold a computational view of mind are really willing to bite this bullet. (Such views have also been elaborately criticized by Mike Johnson and Scott Aaronson — critiques that I have seen no compelling replies to.)

It also seems non-monistic in that it appears impossible to give a plausible account of where a unitary mind is supposed to be found in this picture (e.g. in a picture with discrete computations occurring serially over long distances), except perhaps as a separate, dualist phenomenon that we somehow map onto physically discrete computations occurring over time, which seems to me inelegant and unparsimonious. Not to mention that it gives rise to an explosion of minds, as we can then see minds in a vast set of computations that are somehow causally connected across time and space, with the same computations being included in many distinct minds. This picture is at odds with a monist view that implies a one-to-one correspondence between concrete physical state and concrete mental state — or rather, which sees these two sides as distinct descriptions of the exact same reality.

The question is then how phenomenal binding could occur. You explore a quantum mind hypothesis involving quantum coherence. So what are your reasons for thinking that quantum coherence is necessary for phenomenal binding? Why would, say, electromagnetic fields in a synchronous state not be enough?

DP: Thanks Magnus. Yes, I’m a monistic physicalist. Only the physical is real. However, the intrinsic nature of the physical - the “fire” in the equations - is an open question. A minimum requirement of any scientific account of reality is empirical adequacy. The only empirical (“relating to experience”) evidence that one can access is the content of one’s own consciousness. Consciousness isn’t awareness of one’s surroundings (cf. Perceptual consciousness). The external world isn’t “perceived”; it’s theoretically inferred. If (1) perceptual direct realism is false and (2) sentience is scientifically irreducible to insentience, then the ontology of our most intellectually prestigious theory of the world, scientific materialism, is inconsistent with the entirety of the empirical evidence. This claim sounds wildly extravagant, but I’m not trying to be sensationalist. We need to confront the intellectual scandal at the heart of contemporary science. If physicists and chemists understood the properties of matter and energy, then the empirical evidence shouldn’t exist – none of it – any more than ghosts or fairies. “Observation” shouldn’t exist. You should be a zombie. I should be a zombie. Something is fundamentally wrong with our scientific conceptual scheme. Subjective experience can’t be bracketed off from the rest of natural science and left to philosophers as the “Hard Problem”.

One response to this catastrophic empirical failure is eliminativism. Anti-realists about consciousness recognise that the existence of consciousness is inconsistent with materialism. However, eliminativists suppose that the only way to save monistic physicalism is to disavow their own consciousness. Let’s not investigate eliminative materialism here (cf. Consciousness Realism). For there is another way besides denialism to save the ontological unity of science: non-materialist physicalism. Unlike eliminativism, non-materialist physicalism is empirically adequate. I don’t know if non-materialist physicalism is true. If science can’t account for the empirical evidence, why not abandon monistic physicalism? David Chalmers has explored some kind of naturalistic dualism. However, abandoning physicalism would leave the technological successes of our science-based civilisation wholly unexplained: a miracle in all but name. So instead I prefer to discard the metaphysical assumption that generates the Hard Problem in the first instance.

Oversimplifying, let’s assume that quantum physics is formally complete. According to the intrinsic nature argument, outlined early last century by philosopher Betrand Russell and physicist Arthur Eddington, physics exhaustively describes the structural-relational properties of the universe. Galileo’s “mathematicisation of Nature” has culminated in the triumph of the Standard Model. Yet physics is silent on the intrinsic nature of the physical: physics doesn’t deal in essences, but equations. As it stands, the mathematical apparatus of modern physics can be harnessed to describe either a materialist or idealist ontology. Only an ontology of idealism is consistent with the empirical evidence, i.e. the existence, diverse content and causal efficacy of conscious minds. If we transpose the mathematical apparatus of physics onto an idealist ontology, then the Hard Problem of consciousness doesn’t arise. Within this post-materialist conceptual framework, philosophical talk of the “easy” problems and “hard” problem of consciousness is interpreted as the by-product of bad metaphysics and a false theory of perception. Intuitively, for sure, quantum field theory describes fields of insentience. It’s foot-stampingly obvious. Yet the hypothesis that the world’s fundamental fields are non-experiential isn’t a scientific discovery; rather, it’s a powerful philosophical intuition – an intuition I instinctively share. Yet if history teaches us anything, human intuition can’t be trusted. According to the rival hypothesis of non-materialist physicalism, subjective experience discloses the essence of the physical. For sure, biological minds are special; but they're not ontologically special. The intrinsic nature of the physical doesn’t differ inside and outside one's head. Non-experiential fields of insentience don’t inexplicably metamorphose into fields of sentience inside one's skull. Rather, what does make biological consciousness special is non-psychotic phenomenal binding. Virtual world-making is extremely fitness-enhancing. Crudely, non-psychotic binding is what consciousness is evolutionarily “for”.

Allow me to clarify one point. Yes, as you remark, I believe that our minds disclose a tiny part of the intrinsic nature of the physical. But I wouldn’t say that “mind is identical with the physical”. Most of the world’s fundamental quantum fields are not minds. Digital computers are not minds. The cosmos is not a mind. A hydrogen atom in intergalactic space is not a mind. When dreamlessly asleep, you’re not a mind either. You mention electromagnetic radiation. Let’s assume an incoherent light source. Electromagnetic fields have an instantaneous amplitude and phase that vary randomly with respect to time and position. So electromagnetic fields aren’t minds either (cf. Electromagnetic theories of consciousness). Instead, most of the world’s primordial experience is akin to what philosopher-psychologist William James called “mind dust”. Like pre-scientific animism, traditional panpsychism and idealism have tended to overpopulate the world with minds. If non-materialist physicalism is true, then minds are comparatively sparse, through horrifically dense and proliferating here on Earth.

In recent years, some otherwise hard-nosed scientists have flirted with – or even embraced – consciousness fundamentalism as a solution to the Hard Problem. This conjecture is sometimes known as “constitutive panpsychism” or “constitutive Russellian monism”. “Panpsychism” can suggest property dualism. The intrinsic nature argument is a rejection of any kind of dualism. So I prefer philosopher Grover Maxwell’s term, “non-materialist physicalism”. The world’s fundamental physical properties are experiential.

Both materialist and non-materialist physicalism face a powerful objection. On the face of it, there is a structural mismatch between the properties of our phenomenally bound minds and the properties of the central nervous system as revealed by neuroscience (cf. philosopher Eric Schwitzgebel’s review of Luke Roelof’s "Combining Minds: How to Think about Composite Subjectivity”: Composite Subjectivity). We are packs of decohered, membrane-bound neurons. Regardless of connectivity and connection weights, a pack of discrete neurons separated by synaptic clefts isn’t a phenomenal mind. Admittedly, the structural mismatch isn’t total. When one experiences a cat, distributed neuronal feature-processors (edge-detectors, motion-detectors, colour-mediating neurons, etc) fire in synchrony. But neuroscanning doesn’t detect a feature-bound cat. As you say, Mike Johnson puts the point well in "Principia Qualia", as does Scott Aaronson in his review of neuroscientist Giulio Tononi’s Integrated Information Theory.

My tentative solution to the binding problem is highly counterintuitive. Once again, I should stress: this is only a conjecture, not an affirmation of belief. Recall that physicists working on the foundations of quantum mechanics are mystified by the measurement problem (cf. The Measurement Problem). The wavefunction evolves deterministically in accordance with the Schrödinger equation as a linear superposition of different states. Yet individual observations never find a superposition of multiple values, but instead yield a single measured value. These definite outcomes are found to be in accordance with the Born rule. Why? The one thing that essentially everyone agrees on because it’s blindingly self-evident is that observations and experiments do have definite outcomes – even if (as Everettians propose) non-unique definite outcomes. How else could one even begin to do science if there were no determinate results on which to build theories? And yet (the relativistic generalisation of) the Schrödinger equation makes the existence of determinate results an enigma. Unmodified, the Schrödinger equation tells us that superpositions (“cat states”) should be ubiquitous – which (naively) would make science impossible and (naively) isn't what we observe.

However, two separate questions should be distinguished:
1) How can science explain the existence of phenomenally unified subjects, i.e. “observers” who can make observations of pointer-dial readings, experimental screens and cats within their world-simulations? In other words, how can science explain the existence of vehicles of quasi-perceptual experience like our minds?
2) Why is the subjective content of our observations always determinate, e.g. I observe a live cat, a spin-up electron and so forth?
In my tentative view, the Hard Problem of consciousness, the binding problem in neuroscience and the measurement problem in QM are different facets of the same mystery. Just possibly, they share a solution. Our determinate, phenomenally-bound observations exemplify(!) the superposition principle, not its breakdown. Quantum superpositions (“cat states”) are

(1) individual states, not classical aggregates;
(2) all one knows, and all one will ever know;
(3) make the experience of definite outcomes possible;
(4) what we all are;
(5) the basis of science – classical Newtonian physics as much as quantum field theory.

If you think, “But that’s absurd!”, well, I think so too.
But a better response is, “That’s absurd, and I can show it’s wrong via molecular matter-wave interferometry (cf. Schrödinger neurons: Experimental Protocol).

Either way, I think we should trust the formalism of unitary-only quantum mechanics over perceptual naïve realism. “It is the theory that decides what can be observed”, said Einstein, anticipating Kuhn. Quantum theory determines the very nature of our phenomenally-bound “observations”. If so, then the superposition principle underpins one’s subjective experience of definite, well-defined classical outcomes (“observations”), whether of, say, a phenomenally-bound cat, or the detection of a spin-up electron that has passed through a Stern-Gerlach device, or anything else.

The reader who knows a bit about decoherence (cf. Quantum decoherence) might wonder what professional physicists make of this.
Incredulity, I suspect!
Over the years, quite a few researchers (and New Age mystics and science popularisers) have at least briefly wondered whether two classically impossible forms of holism could be related, i.e. quantum superpositions and the holism of our phenomenally-bound minds. The professional consensus is the idea isn’t a viable solution. Yes, consciousness and binding are mysterious, but introspection of one’s own thought-episodes if nothing else shows that the dynamical timescale of our mental lives is scores if not hundreds of milliseconds. The average cortical neuron fires around 200 times per second. By the same token, our phenomenal world-simulations lag the external environment they track by scores if not hundreds of milliseconds too. By contrast, if we assume the unitary Schrödinger evolution, the effective lifetime of coherent neuronal superpositions in the central nervous system can be only femtoseconds or less. The timescale is wrong by a dozen orders of magnitude or more. Decoherence means the phase angles of the components of individual neuronal superpositions get scrambled to the environment absurdly fast.

So we have reached an impasse.
Classicality is a recipe for micro-experiential zombies.
And decoherence is too powerful, rapid and uncontrollable to allow unified quantum minds. You’re too hot.

Maybe so. But I think this dismissal is too quick. Experimental (dis)confirmation of the “Schrödinger neurons” conjecture is needed – not back-of-an-envelope calculations of decoherence timescales followed by claims of a reductio ad absurdum: Max Tegmark's rebuttal of quantum mind is often cited (cf. "The Importance of Quantum Decoherence in Brain Processes). Recall that according to the intrinsic nature argument, experience discloses the essence of the physical. The intrinsic nature argument isn’t focused on dynamical timescales, but rather on the intrinsic, experiential nature of individual quantum states. If classical physics were true, then non-materialist physicalism would entail we are micro-experiential zombies. Classical physics is false. So instead, we are quantum minds running subjectively classical world-simulations. Or so I reckon:
Quantum Mind

Let’s step back a moment. What would an empirically successful post-materialist science entail?
Any new scientific paradigm must (1) explain all the empirical successes of the old theory, (2) resolve its failures and anomalies and (3) make truly novel, precise and experimentally falsifiable predictions that can (dis)confirm the theory to the satisfaction of proponents and critics alike. Non-materialist physicalism satisfies these criteria. If the phenomenal binding of experience into perceptual objects and definite outcomes is not a classical phenomenon, then the interference signature will tell us. Rephrased, this is an empirical question to be resolved experimentally by the normal methods of science.
I don’t know the answer; I’m highly curious.

MV: So it is because only quantum coherent states could constitute the “ontological unity” of a unitary, “bound” mind. Decoherent states are not and could not, on your view, be ontologically unitary in the required sense?

DP: Yes. Let’s assume, provisionally, non-materialist physicalism: experience discloses the intrinsic nature of the physical. Non-materialist physicalism entails that all and only the world’s fundamental physical properties are experiential. So what’s it like, subjectively, to instantiate coherent superpositions of electric current running in opposite directions, or macromolecules in a superposition of two distinct locations – i.e. classically impossible “Schrödinger kittens” that experimentalists have created? Most relevantly here, what’s it like, subjectively, to instantiate coherent superpositions of neuronal feature-processors that temporally coarse-grained neuroscanning identifies as firing synchronously when you experience a perceptual object? Is it like experiencing, say, a cat? I don’t know. The obvious answer is, “Nothing at all!” because the effective sub-femtosecond lifetime of neuronal superpositions is too short. But if it’s not like anything to instantiate coherent superpositions of neuronal feature-processors, then non-materialist physicalism is false and the Hard Problem returns. Superpositions are individual states – i.e. fundamental physical features of the world – not mere unbound aggregates of classical mind-dust. On this story, decoherence (cf. Quantum decoherence) explains phenomenal unbinding. If we are looking for a perfect structural match between the formalism of physics and phenomenology, the sub-femto realm is where we’ll find it.

Schrödinger himself was a perceptual direct realist – expressly so. Schrödinger speaks of the health status of his eponymous cat being settled by “direct observation”. But as far as I can tell, only the vehicle of neuronal superpositions allows subjectively classical content – our everyday world of experience.

One critical response might run as follows. Suppose for the sake of argument that non-materialist physicalism is true. Suppose that "dynamical collapse" theories are false and neuronal superpositions are both real and experiential. Their "ontological unity" is worthless; such fleeting superpositions yield only psychotic binding, mere functionless "noise", not the promised perfect structural match  Their existence doesn't explain the well-ordered, law-like virtual world of one's everyday experience.

I'd respond by invoking Zurek. More selection pressure in Zurek's sense (cf. Quantum Darwinism) plays out over every second of our existence than over four billion years of natural selection as conceived by Darwin. Such ferocious selection pressure explains the emergence from quantum bedrock of gross, dynamically stable patterns – the quasi-classical, seemingly decohered neurons suggested by today's crude tools of neuroscanning.
I'd love to read a paper "Quantum Darwinism in the CNS".
Sadly, I'm not capable of writing it.

Let's assume wavefunction realism. From a qubit to Schrödinger’s cat, an ignorance interpretation of the wavefunction doesn't work. To use the simplest example of a qubit, it's not the case that an electron is in a state of being either spin-up or spin-down but we don't know which. Rather, it's in a coherent superposition of both states simultaneously. Likewise with short-lived neuronal superpositions that have vastly more components: coherent superpositions are individual states. If non-materialist physicalism is true, then we are dealing with superpositions of micro-experiences. But by hypothesis, the superposition principle still holds – we're just transposing the formalism, not adding to it.

If unitary-only quantum mechanics is correct, then the entire multiverse exists as a gigantic superposition. Decoherence is never totally complete. So the multiverse does have an ontological unity. But neither the multiverse nor a dreamlessly asleep brain is a mind. How come? Well, if decoherence weren't often functionally suppressed, then e.g. digital computers couldn't work. Despite their quantum hardware, which depends on quantum physics, their operation depends on effectively decohered classical bits. Both decohered neurons and the decohered bits of a classical Turing machine are the recipe for micro-experiential zombies.

Anyhow, assume that our minds are indeed made of individual “cat states”. One should always say that the "effective" lifetime of neuronal superpositions is femtoseconds or less because it’s misleading to say such superpositions are literally destroyed by decoherence. Instead, their components get decoupled and acquire phases from their immediate extraneural surroundings. Environmentally-induced decoherence – the scrambling of phase coherence to the environment – turns a coherent quantum superposition into effectively decohered mind-dust. When dreamlessly asleep, we are effectively just packs of neurons.

MV: So in other words, you are essentially saying that binding/unity between decohered states is ultimately no more tenable than binding/unity between, say, two billiard balls separated by a hundred miles? Because they are in a sense similarly ontologically separate?

DP: Yes! Technically, pedantically, quantum entanglement is pervasive in Nature. But this isn't why believers in the emergence of digital minds think that phenomenally-bound consciousness will arise in classical digital computers.

In Newtonian physics, classicality is fundamental. In no-collapse quantum mechanics, classicality is derived – or rather the decoherence program attempts to show how it can be derived.

Digital computing depends on effectively classical, decohered individual bits of information, whether as implemented in Turing's original tape set-up, a modern digital computer, or indeed if the world's population of skull-bound minds agree to participate in an experiment to see if a global mind can emerge from a supposed global brain.

One can't create perceptual objects, let alone unified minds, from classical mind-dust even if strictly the motes of decohered "dust" are only effectively classical, i.e. phase information has leaked away into the environment. If the 1s and 0s of a digital computer are treated as discrete micro-experiential pixels, then when running a program, we don't need to consider the possibility of coherent superpositions of 1s and 0s / micro-experiences. If the bits weren't effectively classical and discrete, then the program wouldn't execute.

Tomorrow's artificial quantum computers are different. But they aren't what believers in the emergence of digital minds envisage.

MV: So to summarize, your argument is roughly the following:
(1) observed phenomenal binding, or a unitary mind, combined with
(2) an empirically well-motivated monistic physicalism, means that
(3) we must look for a unitary physical state as the “mediator”, or rather the physical description, of mind [since the ontological identity from (2) implies that the ontological unity from (1) must be paralleled in our physical description], and it seems that
(4) only quantum coherent states could truly fit the bill of such ontological unity in physical terms.

DP: 1 to 4, yes!

MV: Cool. And in step 4 in particular, to spell that out more clearly, the reasoning is roughly that classical states are effectively (spatiotemporally) serial, discrete, disconnected, etc. Quantum coherent states, in contrast, are a connected, unitary, individual whole.

Classical bits in a sense belong to disjoint “ontological sets”, whereas qubits belong to the same “ontological set” (as I’ve tried to illustrate somewhat clumsily below, and in line with Seth Lloyd’s quote above).
Is that a fair way to put it?

DP: Yes!
I sometimes say, "Who will play Mendel to Zurek's Darwin is unknown." If experience discloses the intrinsic nature of the physical, i.e. if non-materialist physicalism is true, then we must necessarily consider the nature of experience at what are intuitively absurdly short timescales in the CNS. At sufficiently fine-grained temporal resolutions, we can't just assume the existence of decohered macromolecules, neurotransmitters, receptors, membrane-bound neurons (etc) – they are weakly emergent, dynamically stable patterns of "cat states". These high-level patterns must be derived from quantum bedrock – which of course I haven't done. All I've done is make a philosophical conjecture that (1) quantum coherence mediates the phenomenal unity of our minds; and (2) quantum Darwinism (cf. Quantum Darwinism) offers a ludicrously powerful selection-mechanism for sculpting what would otherwise be mere phenomenally-bound "noise".

MV: That’s very interesting. This has certainly helped advance my own understanding of your view a good deal. I guess it’s worth stressing that you do not claim this to be any more than a hypothesis, while you at the same time admit that you have a hard time seeing how alternative accounts could conceivably explain phenomenal binding. Moreover, it’s worth stressing that the conjecture resulting from your line of reasoning above is in fact, as you noted, a falsifiable one — a rare distinction for a theory of consciousness! (Arguably itself worthy of a participation trophy.) Given recent attention to and progress in quantum biology, do you expect this empirical matter to be settled soon?

DP: Yes, it's a hypothesis, no more. There are probably more elegant ways of (dis)confirming the hypothesis than the technically demanding experiment I discuss. I'm not especially optimistic about timescales of (dis)confirmation. The fact someone has a potentially falsifiable theory doesn't excite me if I feel sure their hypothesis is false. I'm sure most people think the same about my ideas. I'm not trying to evangelise – not very energetically at any rate. I'd feel very differently if I had a weird hypothesis that entailed digital zombies were really sentient...

MV: A more general point to note is that skepticism about digital sentience need not be predicated on the conjecture you presented above, as there are other theories of mind — not necessarily involving quantum coherence — that also imply that digital computers are unable to mediate a conscious mind (including some of the theories hinted at above, and perhaps other, more recent theories). For example, one may accept steps 1-3 in the argument above, and be more agnostic in step 4, while still considering contemporary digital computers highly unlikely to be capable of mediating a unitary mind. Okay, having said all that, let’s now move on to a slightly different issue. Beyond digital sentience in particular, you have also expressed skepticism regarding artificial sentience more generally (i.e. non-digital artificial sentience). Can you explain the reasons for this skepticism?

DP: Well, aeons of posthuman biological minds probably lie ahead. They'll be artificial – genetically rewritten, AI-augmented, most likely superhumanly blissful, but otherwise inconceivably alien to Darwinian primitives. My scepticism is about the supposed emergence of minds in classical information processors - whether programmable digital computers, classically parallel connectionist systems or anything else. What about inorganic quantum minds? Well, I say a bit more here:
Anhedonic quantum computers

A pleasure-pain axis has been so central to our experience that sentience in everything from worms to humans is sometimes misdefined in terms of the capacity to feel pleasure and pain. But essentially, I see no reason to believe that such (hypothetical) phenomenally bound consciousness in future inorganic quantum computers will support a pleasure-pain axis any more than, say, the taste of garlic.
In view of our profound ignorance of physical reality, however, I'm cautious: this is just my best guess!

MV: Interesting. You note that you see no reason to believe that such systems would have a pleasure-pain axis. But what about the argument that pain has proven exceptionally adaptive over the course of biological evolution, and might thus plausibly prove adaptive in future forms of evolution as well (assuming things won't necessarily be run according to civilized values)?  

DP: Oh, I think the pleasure-pain axis – or rather one half of it – will endure as long as life itself. Hedonic tone will encephalise (what would otherwise be) hedonically neutral experience in increasingly unnatural biological minds. We can't even begin to know what posthumans will be happy "about". But currently I can't see any reason to suppose hedonic tone (or the taste of garlic) could be instantiated in inorganic quantum computers. I don't think we'll be able to "mind meld"  our reward circuitry with inorganic quantum computers and thereby confer hedonic tone in the way that the encephalization of emotion lends emotional colour to our neocortical virtual worlds. So the question of whether pleasure (or pain) would be adaptive for such devices doesn't arise. If (a big "if") the quantum-theoretic version of non-materialism physicalism is true, then subjectively it's like something to be an inorganic computer, just as it's like something subjectively to be superfluid helium – a nonbiological macro-quale. But out of the zillions of state-spaces of experience, why expect the state-space of phenomenally-bound experience that inorganic quantum computers hypothetically support will include hedonic tone? My guess is that futuristic quantum computers will instantiate qualia for which humans have no name nor conception and with no counterpart in biological minds.

MV: It seems there are good reasons to be skeptical that states of suffering would emerge in any randomly created state, as you hint, but what if someone deliberately tried to create such states in an inorganic quantum computer?

DP: I'll be surprised if phenomenally bound pain and pleasure don't critically depend on the valence properties of carbon and liquid water.

MV: What about hypothetical future computers built from biological neurons?

DP: Artificial organic neuronal networks are perfectly feasible. Unlike silicon-based "neural networks" — a misnomer in my view — certain kinds of artificial organic neuronal networks could indeed suffer. Consider the reckless development of "mini-brains".

MV: Yeah, it should be uncontroversial that such developments entail serious risks.
Okay, David. What you have said here certainly provides much food for thought. Hopefully it will encourage further reflection on these things. Thanks a lot for exploring these issues, and not least for all your work and your dedication to reducing the suffering of all sentient beings.

DP: Thank you, Magnus. You’re very kind. May I just add a recommendation? Anyone who hasn’t yet done so should read your superb Suffering-Focused Ethics (2020).

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