Some Definitions

Both physics and philosophy are jargon-ridden. So let’s first define some key concepts.


Both “consciousness” and “physical” are contested terms. Accurately if inelegantly, consciousness may be described following Nagel (“What is it like to be a bat?”) as the subjective what-it’s-like-ness of experience. Academic philosophers term such self-intimating “raw feels” “qualia” – whether macro-qualia or micro-qualia. The minimum unit of consciousness (or “psychon”, so to speak) has been variously claimed to be the entire universe, a person, a sub-personal neural network, an individual neuron, or the most basic entities recognised by quantum physics. In The Principles of Psychology (1890), American philosopher and psychologist William James christened these phenomenal simples “primordial mind-dust“. This paper conjectures that (1) our minds consist of ultra-rapidly decohering neuronal superpositions in strict accordance with unmodified quantum physics without the mythical “collapse of the wavefunction”; (2) natural selection has harnessed the properties of these neuronal superpositions so our minds run phenomenally-bound world-simulations; and (3) predicts that with enough ingenuity the non-classical interference signature of these conscious neuronal superpositions will be independently experimentally detectable (see 6 below) to the satisfaction of the most incredulous critic.


The “physical” may be contrasted with the supernatural or the abstract and – by dualists and epiphenomenalists, with the mental. The current absence of any satisfactory “positive” definition of the physical leads many philosophers of science to adopt instead the “via negativa“. Thus some materialists have sought stipulatively to define the physical in terms of an absence of phenomenal experience. Such a priori definitions of the nature of the physical are question-begging.


Physicalism” is sometimes treated as the formalistic claim that the natural world is exhaustively described by the equations of physics and their solutions. Beyond these structural-relational properties of matter and energy, the term “physicalism” is also often used to make an ontological claim about the intrinsic character of whatever the equations describe. This intrinsic character, or metaphysical essence, is typically assumed to be non-phenomenal. “Strawsonian physicalists” (cf. “Consciousness and Its Place in Nature: Does Physicalism Entail Panpsychism?”) dispute any such assumption. Traditional reductive physicalism proposes that the properties of larger entities are determined by properties of their physical parts. If the wavefunction monism of post-Everett quantum mechanics assumed here is true, then the world does not contain discrete physical parts as understood by classical physics.


Materialism” is the metaphysical doctrine that the world is made of intrinsically non-phenomenal “stuff”. Materialism and physicalism are often treated as cousins and sometimes as mere stylistic variants – with “physicalism” used as a nod to how bosonic fields, for example, are not matter. “Physicalistic materialism” is the claim that physical reality is fundamentally non-experiential and that the natural world is exhaustively described by the equations of physics and their solutions.


Panpsychism” is the doctrine that the world’s fundamental physical stuff also has primitive experiential properties. Unlike the physicalistic idealism explored here, panpsychism doesn’t claim that the world’s fundamental physical stuff is experiential.


Epiphenomenalism” in philosophy of mind is the view that experience is caused by material states or events in the brain but does not itself cause anything; the causal efficacy of mental agency is an illusion.


For our purposes, “idealism” is the ontological claim that reality is fundamentally experiential. This use of the term should be distinguished from Berkeleyan idealism, and more generally, from subjective idealism, i.e. the doctrine that only mental contents exist: reality is mind-dependent. One potential source of confusion of contemporary scientific idealism with traditional philosophical idealism is the use by inferential realists in the theory of perception of the term “world-simulation”. The mind-dependence of one’s phenomenal world-simulation, i.e. the quasi-classical world of one’s everyday experience, does not entail the idealist claim that the mind-independent physical world is intrinsically experiential in nature – a far bolder conjecture that we nonetheless tentatively defend here.


Physicalistic idealism” is the non-materialist physicalist claim that reality is fundamentally experiential and that the natural world is exhaustively described by the equations of physics and their solutions: more specifically, by the continuous, linear, unitary evolution of the universal wavefunction of post-Everett quantum mechanics. The “decoherence program” in contemporary theoretical physics aims to show in a rigorously quantitative manner how quasi-classicality emerges from the unitary dynamics.


Monism” is the conjecture that reality consists of a single kind of “stuff” – be it material, experiential, spiritual, or whatever. Wavefunction monism is the view that the universal wavefunction mathematically represents, exhaustively, all there is in the world. Strictly speaking, wavefunction monism shouldn’t be construed as the claim that reality literally consists of a certain function, i.e. a mapping from some mind-wrenchingly immense configuration space to the complex numbers, but rather as the claim that every mathematical property of the wavefunction except the overall phase corresponds to some property of physical world. “Dualism”, the conjecture that reality consists of two kinds of “stuff”, comes in many flavours: naturalistic and theological; interactionist and non-interactionist; property and ontological. In the modern era, most scientifically literate monists have been materialists. But to describe oneself as both a physicalist and a monistic idealist is not the schizophrenic word-salad it sounds at first blush.


Functionalism” in philosophy of mind is the theory that mental states are constituted solely by their functional role, i.e. by their causal relations to other mental states, perceptual inputs, and behavioural outputs. Functionalism is often associated with the idea of “substrate-neutrality”, sometimes misnamed “substrate-independence”, i.e. minds can be realised in multiple substrates and at multiple levels of abstraction. However, micro-functionalists may dispute substrate-neutrality on the grounds that one or more properties of mind, for example phenomenal binding, functionally implicate the world’s quantum-mechanical bedrock from which the quasi-classical worlds of Everett’s multiverse emerge. Thus this paper will argue that only successive quantum-coherent neuronal superpositions at naively preposterously short time-scales can explain phenomenal binding. Without phenomenal binding, no functionally adaptive classical world-simulations could exist in the first instance.


The “binding problem(10), also called the “combination problem”, refers to the mystery of how the micro-experiences mediated by supposedly discrete and distributed neuronal edge-detectors, motion-detectors, shape-detectors, colour-detectors (etc) can be “bound” into unitary experiential objects (“local” binding) apprehended by a unitary experiential self (“global” binding). Neuroelectrode studies using awake, verbally competent human subjects confirm that neuronal micro-experiences exist. Classical neuroscience cannot explain how they could ever be phenomenally bound.


Mereology” is the theory of the relations between part to whole and the relations between part to part within a whole. Scientifically literate humans find it’s natural and convenient to think of particles, macromolecules or neurons as having their own individual wavefunctions by which they can be formally represented. However, the manifest non-classicality of phenomenal binding means that in some contexts we must consider describing the entire mind-brain via a single wavefunction. Organic minds are not simply the “mereological sum” of discrete classical parts. Organic brains are not simply the “mereological sum” of discrete classical neurons.


Quantum field theory” is the formal, mathematico-physical description of the natural world. The world is made up of the states of quantum fields, conventionally non-experiential in character, that take on discrete values. Physicists use mathematical entities known as “wavefunctions” to represent quantum states. Wavefunctions may be conceived as representing all the possible configurations of a superposed quantum system. Wavefunction(al)s are complex valued functionals on the space of field configurations. Wavefunctions in quantum mechanics are sinusoidal functions with an amplitude (a “measure”) and also a phase. The Schrödinger equation:




describes the time-evolution of a wavefunction. “Coherence” means that the phases of the wavefunction are kept constant between the coherent particles, macromolecules or (hypothetically) neurons, while “decoherence” is the effective loss of ordering of the phase angles between the components of a system in a quantum superposition. Such thermally-induced “dephasing” rapidly leads to the emergence – on a perceptual naive realist story – of classical, i.e. probabilistically additive, behaviour in the central nervous system (“CNS”), and also the illusory appearance of separate, non-interfering organic macromolecules. Hence the discrete, decohered classical neurons of laboratory microscopy and biology textbooks. Unlike classical physics, quantum mechanics deals with superpositions of probability amplitudes rather than of probabilities; hence the interference terms in the probability distribution. Decoherence should be distinguished from dissipation, i.e. the loss of energy from a system – a much slower, classical effect. Phase coherence is a quantum phenomenon with no classical analogue. If quantum theory is universally true, then any physical system such as a molecule, neuron, neuronal network or an entire mind-brain exists partly in all its theoretically allowed states, or configuration of its physical properties, simultaneously in a “quantum superposition“; informally, a “Schrödinger’s cat state”. Each state is formally represented by a complex vector in Hilbert space. Whatever overall state the nervous system is in can be represented as being a superposition of varying amounts of these particular states (“eigenstates”) where the amount that each eigenstate contributes to the overall sum is termed a component. The “Schrödinger equation” is a partial differential equation that describes how the state of a physical system changes with time. The Schrödinger equation acts on the entire probability amplitude, not merely its absolute value. The absolute value of the probability amplitude encodes information about probability densities, so to speak, whereas its phase encodes information about the interference between quantum states. On measurement by an experimenter, the value of the physical quantity in a quantum superposition will naively seem to “collapse” in an irreducibly stochastic manner, with a probability equal to the square of the coefficient of the superposition in the linear combination. If the superposition principle really breaks down in the mind-brain, as traditional Copenhagen positivists still believe, then the central conjecture of this paper is false.


Mereological nihilism“, also known as “compositional nihilism”, is the philosophical position that objects with proper parts do not exist, whether extended in space or in time. Only basic building blocks (particles, fields, superstrings, branes, information, micro-experiences, quantum superpositions, entangled states, or whatever) without parts exist. Such ontological reductionism is untenable if the mind-brain supports macroscopic quantum coherence in the guise of bound phenomenal states because coherent neuronal superpositions describe individual physical states. Coherent superpositions of neuronal feature-detectors cannot be interpreted as classical ensembles of states. Radical ontological reductionism is even more problematic if post-Everett(11) quantum mechanics is correct: reality is exhaustively described by the time-evolution of one gigantic universal wavefunction. If such “wavefunction monism” is true, then talk of how neuronal superpositions are rapidly “destroyed” is just a linguistic convenience because a looser, heavily-disguised coherence persists within a higher-level Schrödinger equation (or its relativistic generalisation) that subsumes the previously tighter entanglement within a hierarchy of wavefunctions, all ultimately subsumed within the universal wavefunction.


Direct realism“, also known as “naive realism”, about perception is the pre-scientific view that the mind-brain is directly acquainted with the external world. In contrast, the “world-simulation model”(12) assumed here treats the mind-brain as running a data-driven simulation of gross fitness-relevant patterns in the mind-independent environment. As an inferential realist, the world-simulationist is not committed per se to any kind of idealist ontology, physicalistic or otherwise. However, s/he will understand phenomenal consciousness as broader in scope compared to the traditional perceptual direct realist. The world-simulationist will also be less confident than the direct realist that we have any kind of pre-theoretic conceptual handle on the nature of the “physical” beyond the formalism of theoretical physics – and our own phenomenally-bound physical consciousness.


“Classical worlds” are what perceptual direct realists call the world. Quantum theory suggests that the multiverse exists in an inconceivably vast cosmological superposition. Yet within our individual perceptual world-simulations, familiar macroscopic objects 1) occupy definite positions (the “preferred basis” problem); 2) don’t readily display quantum interference effects; and 3) yield well-defined outcomes when experimentally probed. Cats are either dead or alive, not dead-and-alive. Or as one scientific populariser puts it, “Where Does All the Weirdness Go?” This paper argues that the answer lies under our virtual noses – though independent physical proof will depend on next-generation matter-wave interferometry. Phenomenally-bound classical world-simulations are the mind-dependent signature of the quantum “weirdness”. Without the superposition principle, no phenomenally-bound classical world-simulations could exist – and no minds. In short, we shouldn’t imagine superpositions of live-and-dead cats, but instead think of superpositions of colour-, shape-, edge- and motion-processing neurons. Thanks to natural selection, the content of our waking world-simulations typically appears classical; but the vehicle of the simulation that our minds run is inescapably quantum. If the world were classical it wouldn’t look like anything to anyone.


A “zombie“, sometimes called a “philosophical zombie” or “p-zombie” to avoid confusion with its lumbering Hollywood cousins, is a hypothetical organism that is materially and behaviourally identical to humans and other organic sentients but which isn’t conscious. Philosophers explore the epistemological question of how each of us can know that s/he isn’t surrounded by p-zombies. Yet we face a mystery deeper than the ancient sceptical Problem of Other Minds. If our ordinary understanding of the fundamental nature of matter and energy as described by physics is correct, and if our neurons are effectively decohered classical objects as suggested by standard neuroscience, then we all ought to be zombies. Following David Chalmers, this is called the Hard Problem of consciousness.


Non-Materialist Physicalism: An experimentally Testable Conjecture by David Pearce

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