Hard problem of consciousness

. . .even when we have explained the performance of all the cognitive and behavioral functions in the vicinity of experience—perceptual discrimination, categorization, internal access, verbal report—there may still remain a further unanswered question: Why is the performance of these functions accompanied by experience?

— David Chalmers, Facing up to the problem of consciousness

The problem of consciousness, Chalmers argues, is two problems: the easy problems and the hard problem.

The easy problems are amenable to reductive inquiry. They are a logical consequence of lower level facts about the world, similar to how a clock's ability to tell time is a logical consequence of its clockwork and structure, or a hurricane is a logical consequence of the structures and functions of certain weather patterns. A clock, a hurricane, and the easy problems, are all the sum of their parts (as are most things).[24]

The easy problems relevant to consciousness concern mechanistic analysis of the neural processes that accompany behaviour. Examples of these include how sensory systems work, how sensory data is processed in the brain, how that data influences behaviour or verbal reports, the neural basis of thought and emotion, and so on. They are problems that can be analyzed through "structures and functions".[24]

Chalmers' use of the word easy is "tongue-in-cheek".[29] As Steven Pinker puts it, they are about as easy as going to Mars or curing cancer. "That is, scientists more or less know what to look for, and with enough brainpower and funding, they would probably crack it in this century."[30]

The hard problem, in contrast, is the problem of why and how those processes are accompanied by experience.[3] It may further include the question of why these processes are accompanied by this or that particular experience, rather than some other kind of experience. In other words, the hard problem is the problem of explaining why certain mechanisms are accompanied by conscious experience.[24] For example, why should neural processing in the brain lead to the felt sensations of, say, feelings of hunger? And why should those neural firings lead to feelings of hunger rather than some other feeling (such as, for example, feelings of thirst)?

Chalmers argues that it is conceivable that the relevant behaviours associated with hunger, or any other feeling, could occur even in the absence of that feeling. This suggests that experience is irreducible to physical systems such as the brain. This is the topic of the next section.

Chalmers believes that the hard problem is irreducible to the easy problems: solving the easy problems will not lead to a solution to the hard problems. This is because the easy problems pertain to the causal structure of the world while the hard problem pertains to consciousness, and facts about consciousness include facts that go beyond mere causal or structural description.

For example, suppose someone were to stub their foot and yelp. In this scenario, the easy problems are mechanistic explanations that involve the activity of the nervous system and brain and its relation to the environment (such as the propagation of nerve signals from the toe to the brain, the processing of that information and how it leads to yelping, and so on). The hard problem is the question of why these mechanisms are accompanied by the feeling of pain, or why these feelings of pain feel the particular way that they do. Chalmers argues that facts about the neural mechanisms of pain, and pain behaviours, do not lead to facts about conscious experience. Facts about conscious experience are, instead, further facts, not derivable from facts about the brain.[24]

The hard problem is often illustrated by appealing to the logical possibility of inverted visible spectra. If there is no logical contradiction in supposing that one's colour vision could be inverted, it follows that mechanistic explanations of visual processing do not determine facts about what it is like to see colours.

An explanation for all of the relevant physical facts about neural processing would leave unexplained facts about what it is like to feel pain. This is in part because functions and physical structures of any sort could conceivably exist in the absence of experience. Alternatively, they could exist alongside a different set of experiences. For example, it is logically possible for a perfect replica of Chalmers to have no experience at all, or for it to have a different set of experiences (such as an inverted visible spectrum, so that the blue-yellow red-green axes of its visual field are flipped).

The same cannot be said about clocks, hurricanes, or other physical things. In those cases, a structural or functional description is a complete description. A perfect replica of a clock is a clock, a perfect replica of a hurricane is a hurricane, and so on. The difference is that physical things are nothing more than their physical constituents. For example, water is nothing more than H₂O molecules, and understanding everything about H₂O molecules is to understand everything there is to know about water. But consciousness is not like this. Knowing everything there is to know about the brain, or any physical system, is not to know everything there is to know about consciousness. So consciousness, then, must not be purely physical.[24]

Chalmers's idea contradicts physicalism (sometimes labelled materialism). This is the view that everything that exists is a physical or material thing, so everything can be reduced to microphysical things (such as subatomic particles and the interactions between them). For example, a desk is a physical thing, because it is nothing more than a complex arrangement of a large number of subatomic particles interacting in a certain way. According to physicalism, everything can be explained by appeal to its microphysical constituents, including consciousness. Chalmers's hard problem presents a counterexample to this view, since it suggests that consciousness cannot be reductively explained by appealing to its microphysical constituents. So if the hard problem is a real problem then physicalism must be false, and if physicalism is true then the hard problem must not be a real problem.

Though Chalmers rejects physicalism, he is still a naturalist.[24]

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The hard problem of consciousness has scholarly antecedents considerably earlier than Chalmers. Chalmers himself notes that "a number of thinkers in the recent and distant past" have "recognised the particular difficulties of explaining consciousness."[31] He states that all his original 1996 paper contributed to the discussion was "a catchy name, a minor reformulation of philosophically familiar points".[31]

Among others, thinkers who have made arguments similar to Chalmers' formulation of the hard problem include Isaac Newton,[32] John Locke,[33] Gottfried Wilhelm Leibniz,[34][32] John Stuart Mill,[35] and Thomas Henry Huxley.[36][32] Likewise, Asian philosophers like Dharmakirti and Guifeng Zongmi discussed the problem of how consciousness arises from unconscious matter.[32][37][38][39]

Philosopher Raamy Majeed argued in 2016 that the hard problem is associated with two "explanatory targets":[40]

1. [PQ] Physical processing gives rise to experiences with a phenomenal character.
2. [Q] Our phenomenal qualities are thus-and-so.

The first fact concerns the relationship between the physical and the phenomenal (i.e., how and why are some physical states felt states), whereas the second concerns the very nature of the phenomenal itself (i.e., what does the felt state feel like?).

Wolfgang Fasching argues that the hard problem is not about qualia, but about the what-it-is-like-ness of experience in Nagel's sense—about the givenness of phenomenal contents:

Today there is a strong tendency to simply equate consciousness with the qualia. Yet there is clearly something not quite right about this. The "itchiness of itches" and the "hurtfulness of pain" are qualities we are conscious of. So philosophy of mind tends to treat consciousness as if it consisted simply of the contents of consciousness (the phenomenal qualities), while it really is precisely consciousness of contents, the very givenness of whatever is subjectively given. And therefore the problem of consciousness does not pertain so much to some alleged "mysterious, nonpublic objects", i.e. objects that seem to be only "visible" to the respective subject, but rather to the nature of "seeing" itself (and in today’s philosophy of mind astonishingly little is said about the latter).[41]

The mind–body problem is the problem of how the mind and the body relate. The mind body problem is more general than the hard problem of consciousness, since it is the problem of discovering how the mind and body relate in general, thereby implicating any theoretical framework that broaches the topic. The hard problem, in contrast, is often construed as a problem uniquely faced by physicialist or materialist theories of mind.

The philosopher Thomas Nagel posited in his 1974 paper "What Is It Like to Be a Bat?" that experiences are essentially subjective (accessible only to the individual undergoing them—i.e., felt only by the one feeling them), while physical states are essentially objective (accessible to multiple individuals). So he argued we have no idea what it could mean to claim that an essentially subjective state just is an essentially non-subjective state (i.e., that a felt state is nothing but a functional state). In other words, we have no idea of what reductivism amounts to.[28] He believes "every subjective phenomenon is essentially connected with a single point of view, and it seems inevitable that an objective, physical theory will abandon that point of view."[28]

In 1983, the philosopher Joseph Levine proposed that there is an explanatory gap between our understanding of the physical world and our understanding of consciousness.[42]

Levine's disputes that conscious states are reducible to neuronal or brain states. He uses the example of pain (as an example of a conscious state) and its reduction to the firing of c-fibers (a kind of nerve cell). The difficulty is as follows: even if consciousness is physical, it is not clear which physical states correspond to which conscious states. The bridges between the two levels of description will be contingent, rather than necessary. This is significant because in most contexts, relating two scientific levels of descriptions (such as physics and chemistry) is done with the assurance of necessary connections between the two theories (for example, chemistry follows with necessity from physics).[43]

Levine illustrates this with a thought experiment: Suppose that humanity were to encounter an alien species, and suppose it is known that the aliens do not have any c-fibers. Even if one knows this, it is not obvious that the aliens do not feel pain: that would remain an open question. This is because the fact that aliens do not have c-fibers does not entail that they do not feel pain (in other words, feelings of pain do not follow with logical necessity from the firing of c-fibers). Levine thinks such thought experiments demonstrate an explanatory gap between consciousness and the physical world: even if consciousness is reducible to physical things, consciousness cannot be explained in terms of physical things, because the link between physical things and consciousness is contingent link.[43]

Levine does not think that the explanatory gap means that consciousness is not physical: he is open to the idea that the explanatory gap is only an epistemological problem for physicalism.[43] In contrast, Chalmers thinks that the hard problem of consciousness does show that consciousness is not physical.[24]

Philosophical zombies are a thought experiment commonly used in discussions of the hard problem.[44][45] They are hypothetical beings physically identical to humans but that lack conscious experience.[46] Philosophers such as Chalmers, Joseph Levine, and Francis Kripke take zombies as impossible within the bounds of nature but possible within the bounds of logic.[47] This would imply that facts about experience are not logically entailed by the "physical" facts. Therefore, consciousness is irreducible. In Chalmers' words, "after God (hypothetically) created the world, he had more work to do."[48] Daniel Dennett, a philosopher of mind, criticised the field's use of "the zombie hunch" which he deems an "embarrassment"[49] that ought to "be dropped like a hot potato".[26]

The knowledge argument, also known as Mary's Room, is another common thought experiment: A hypothetical neuroscientist named Mary has lived her whole life in a black-and-white room and has never seen colour before. She also happens to know everything there is to know about the brain and colour perception.[50] Chalmers believes[48] that when Mary sees the colour red for the first time, she gains new knowledge — the knowledge of "what red looks like" — which is distinct from, and irreducible to, her prior physical knowledge of the brain or visual system. A stronger form of the knowledge argument[50] claims not merely that Mary would lack subjective knowledge of "what red looks like," but that she would lack knowledge of an objective fact about the world: namely, "what red looks like," a non-physical fact that can be learned only through direct experience (qualia). Others, such as Thomas Nagel, take a "physicalist" position, disagree with the argument in its stronger and/or weaker forms.[50] For example, Nagel put forward a "speculative proposal" of devising a language that could "explain to a person blind from birth what it is like to see."[28] The knowledge argument implies that such a language could not exist.

Type-A materialism (also known as reductive materialism or a priori physicalism) is view characterized by a commitment to physicalism and a full rejection of the hard problem. By this view, the hard problem either does not exist or is just another easy problem, because every fact about the mind is a fact about the performance of various functions or behaviours. So, once all the relevant functions and behaviours have been accounted for, there will not be any facts left over in need of explanation.[53] Thinkers who subscribe to type-A materialism include Paul and Patricia Churchland, Daniel Dennett, Keith Frankish, and Thomas Metzinger.

Some type-A materialists believe in the reality of phenomenal consciousness but believe it is nothing extra in addition to certain functions or behaviours. This view is sometimes referred to as strong reductionism.[43][59] Other type-A materialists may reject the existence of phenomenal consciousness entirely. This view is referred to as eliminative materialism or illusionism.[60][61][62]

Many philosophers have disputed that there is a hard problem of consciousness distinct from what Chalmers calls the easy problems of consciousness. Some among them, who are sometimes termed strong reductionists, hold that phenomenal consciousness (i.e., conscious experience) does exist but that it can be fully understood as reducible to the brain.[43]

Broadly, strong reductionists accept that conscious experience is real but argue it can be fully understood in functional terms as an emergent property of the material brain.[43] In contrast to weak reductionists (see above), strong reductionists reject ideas used to support the existence of a hard problem (that the same functional organization could exist without consciousness, or that a blind person who understood vision through a textbook would not know everything about sight) as simply mistaken intuitions.[43][59]

A notable family of strong reductionist accounts are the higher-order theories of consciousness.[63][43] In 2005, the philosopher Peter Carruthers wrote about "recognitional concepts of experience", that is, "a capacity to recognize [a] type of experience when it occurs in one's own mental life," and suggested that such a capacity could explain phenomenal consciousness without positing qualia.[64] On the higher-order view, since consciousness is a representation, and representation is fully functionally analyzable, there is no hard problem of consciousness.[43]

The philosophers Glenn Carruthers and Elizabeth Schier said in 2012 that the main arguments for the existence of a hard problem—philosophical zombies, Mary's room, and Nagel's bats—are only persuasive if one already assumes that "consciousness must be independent of the structure and function of mental states, i.e. that there is a hard problem." Hence, the arguments beg the question. The authors suggest that "instead of letting our conclusions on the thought experiments guide our theories of consciousness, we should let our theories of consciousness guide our conclusions from the thought experiments."[65]

The philosopher Massimo Pigliucci argued in 2013 that the hard problem is misguided, resulting from a "category mistake".[16] He said: "Of course an explanation isn't the same as an experience, but that's because the two are completely independent categories, like colors and triangles. It is obvious that I cannot experience what it is like to be you, but I can potentially have a complete explanation of how and why it is possible to be you."[16]

In 2017, the philosopher Marco Stango, in a paper on John Dewey's approach to the problem of consciousness (which preceded Chalmers' formulation of the hard problem by over half a century), noted that Dewey's approach would see the hard problem as the consequence of an unjustified assumption that feelings and functional behaviors are not the same physical process: "For the Deweyan philosopher, the 'hard problem' of consciousness is a 'conceptual fact' only in the sense that it is a philosophical mistake: the mistake of failing to see that the physical can be had as an episode of immediate sentiency."[66]

The philosopher Thomas Metzinger likens the hard problem of consciousness to vitalism, a formerly widespread view in biology which was not so much solved as abandoned.[67] Brian Jonathan Garrett has also argued that the hard problem suffers from flaws analogous to those of vitalism.[68]

The philosopher Peter Hacker argues that the hard problem is misguided in that it asks how consciousness can emerge from matter, whereas in fact sentience emerges from the evolution of living organisms.[69] He states: "The hard problem isn’t a hard problem at all. The really hard problems are the problems the scientists are dealing with. [...] The philosophical problem, like all philosophical problems, is a confusion in the conceptual scheme."[69] Hacker's critique extends beyond Chalmers and the hard problem and is directed against contemporary philosophy of mind and neuroscience more broadly. Along with the neuroscientist Max Bennett, he has argued that most of contemporary neuroscience remains implicitly dualistic in its conceptualizations and is predicated on the mereological fallacy of ascribing psychological concepts to the brain that can properly be ascribed only to the person as a whole.[70] Hacker further states that "consciousness studies", as it exists today, is "literally a total waste of time" and that "the conception of consciousness which they have is incoherent".[69]

Eliminative materialism or eliminativism is the view that many or all of the mental states used in folk psychology (i.e., common-sense ways of discussing the mind) do not, upon scientific examination, correspond to real brain mechanisms.[60] According the 2020 PhilPapers survey, 4.51% of philosophers surveyed subscribe to eliminativism.[51]

While Patricia Churchland and Paul Churchland have famously applied eliminative materialism to propositional attitudes, philosophers including Daniel Dennett, Georges Rey, and Keith Frankish have applied it to qualia or phenomenal consciousness (i.e., conscious experience).[60] On their view, it is mistaken not only to believe there is a hard problem of consciousness, but to believe phenomenal consciousness exists at all.[18][71]

This stance has recently taken on the name of illusionism: the view that phenomenal consciousness is an illusion. The term was popularized by the philosopher Keith Frankish.[61] Frankish argues that "illusionism" is preferable to "eliminativism" for labelling the view that phenomenal consciousness is an illusion. More substantively, Frankish argues that illusionism about phenomenal consciousness is preferable to realism about phenomenal consciousness. He states: "Theories of consciousness typically address the hard problem. They accept that phenomenal consciousness is real and aim to explain how it comes to exist. There is, however, another approach, which holds that phenomenal consciousness is an illusion and aims to explain why it seems to exist."[18] Frankish concludes that illusionism "replaces the hard problem with the illusion problem—the problem of explaining how the illusion of phenomenality arises and why it is so powerful."[18]

The philosopher Daniel Dennett is another prominent figure associated with illusionism. After Frankish published a paper in the Journal of Consciousness Studies titled Illusionism as a Theory of Consciousness,[61] Dennett responded with his own paper with the spin-off title Illusionism as the Obvious Default Theory of Consciousness.[71] Dennett has been arguing for the illusory status of consciousness since early on in his career. For example, in 1979 he published a paper titled On the Absence of Phenomenology (where he argues for the nonexistence of phenomenal consciousness).[72] Similar ideas have been explicated in his 1991 book Consciousness Explained.[73] Dennett argues that the so-called "hard problem" will be solved in the process of solving what Chalmers terms the "easy problems".[15] He compares consciousness to stage magic and its capability to create extraordinary illusions out of ordinary things.[74] To show how people might be commonly fooled into overstating the accuracy of their introspective abilities, he describes a phenomenon called change blindness, a visual process that involves failure to detect scenery changes in a series of alternating images.[75] He accordingly argues that consciousness need not be what it seems to be based on introspection. To address the question of the hard problem, or how and why physical processes give rise to experience, Dennett states that the phenomenon of having experience is nothing more than the performance of functions or the production of behavior, which can also be referred to as the easy problems of consciousness.[15] Thus, Dennett argues that the hard problem of experience is included among—not separate from—the easy problems, and therefore they can only be explained together as a cohesive unit.[74]

Elimativists differ on the role they believe intuitive judgement plays in creating the apparent reality of consciousness. The philosopher Jacy Reese Anthis is of the position that this issue is born of an overreliance on intuition, calling philosophical discussions on the topic of consciousness a form of "intuition jousting".[76] But when the issue is tackled with "formal argumentation" and "precise semantics" then the hard problem will dissolve.[76] The philosopher Elizabeth Irvine, in contrast, can be read as having the opposite view, since she argues that phenomenal properties (that is, properties of consciousness) do not exist in our common-sense view of the world. She states that "the hard problem of consciousness may not be a genuine problem for non-philosophers (despite its overwhelming obviousness to philosophers)."[77]

A complete illusionist theory of consciousness must include the description of a mechanism by which the illusion of subjective experience is had and reported by people. Various philosophers and scientists have proposed possible theories.[78] For example, in his book Consciousness and the Social Brain neuroscientist Michael Graziano advocates what he calls attention schema theory, in which our perception of being conscious is merely an error in perception, held by brains which evolved to hold erroneous and incomplete models of their own internal workings, just as they hold erroneous and incomplete models of their own bodies and of the external world.[79][80]

The main criticisms of eliminative materialism and illusion hinge on the counterintuitive nature of the view. Arguments of this form are called Moorean Arguments. A Moorean argument seeks to undermine the conclusion of an argument by asserting that the negation of that conclusion is more certain than the premises of the argument.[81]

The roots of the Moorean Argument against illusionism extend back to Augustine of Hippo who stated that he could not be deceived regarding his own existence, since the very act of being deceived secures the existence of a being there to be the recipient of that deception.[note 1][82]

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Descartes' Discourse on the Method/Part 4

In the Early-Modern era, these arguments were repopularized by René Descartes, who coined the now famous phrase "Je pense, donc je suis" ("I think, therefore I am").[83] Descartes argued that even if he was maximally deceived (because, for example, an evil demon was manipulating all his senses) he would still know with certainty that his mind exists, because the state of being deceived requires a mind as a prerequisite.[84]

This same general argumentative structure is still in use today. For example, in 2002 David Chalmers published an explicitly Moorean argument against illusionism. The argument goes like this: The reality of consciousness is more certain than any theoretical commitments (to, for example, physicalism) that may be motivating the illusionist to deny the existence of consciousness. The reason for this is because we have direct "acquaintance" with consciousness, but we do not have direct acquaintance with anything else (including anything that could inform our beliefs in consciousness being an illusion). In other words: consciousness can be known directly, so the reality of consciousness is more certain than any philosophical or scientific theory that says otherwise.[85] Chalmers concludes that "there is little doubt that something like the Moorean argument is the reason that most people reject illusionism and many find it crazy."[86]

Eliminative materialism and illusionism have been the subject of criticism within the popular press. One highly cited example comes from the philosopher Galen Strawson who wrote an article in the New York Review of Books titled The Consciousness Deniers. In it, Strawson describes illusionism as the "silliest claim ever made", next to which "every known religious belief is only a little less sensible than the belief that the grass is green."[87] Another notable example comes from Christof Koch (a neuroscientist and one of the leading proponents of Integrated Information Theory) in his popular science book The Feeling of Life Itself. In the early pages of the book, Koch describes eliminativism as the "metaphysical counterpart to Cotard's syndrome, a psychiatric condition in which patients deny being alive."[88] Koch takes the prevalence of eliminativism as evidence that "much of twentieth-century analytic philosophy has gone to the dogs".[89]

Type-B Materialism, also known as Weak Reductionism or A Posteriori Physicalism, is the view the hard problem stems from human psychology, and is therefore not indicative of a genuine ontological gap between consciousness and the physical world.[43] Like Type-A Materialists, Type-B Materialists are committed to physicalism. Unlike Type-A Materialists, however, Type-B Materialists do accept inconceivability arguments often cited in support of the hard problem, but with a key caveat: that inconceivability arguments give us insight only into how the human mind tends to conceptualize the relationship between mind and matter, but not into what the true nature of this relationship actually is.[43][59] According to this view, there is a gap between two ways of knowing (introspection and neuroscience) that will not be resolved by understanding all the underlying neurobiology, but still believe that consciousness and neurobiology are one and the same in reality.[43]

While Type-B Materialists all agree that intuitions about the hard problem are psychological rather than ontological in origin, they differ as to whether our intuitions about the hard problem are innate or culturally conditioned. This has been dubbed the "hard-wired/soft-wired distinction."[90][91] In relation to Type-B Materialism, those who believe that our intuitions about the hard problem are innate (and therefore common to all humans) subscribe to the "hard-wired view".[91] Those that believe our intuitions are culturally conditioned subscribe to the "soft-wired view". Unless otherwise specified, the term 'Type-B Materialism' refers to the hard-wired view.[91]

Notable philosophers who subscribe to Type-B Materialism include David Papineau,[92] Joseph Levine and [93] Janet Levine.[55]

Joseph Levine (who formulated the notion of the explanatory gap) states: "The explanatory gap argument doesn't demonstrate a gap in nature, but a gap in our understanding of nature."[93] He nevertheless contends that full scientific understanding will not close the gap,[43] and that analogous gaps do not exist for other identities in nature, such as that between water and H₂O.[94] The philosophers Ned Block and Robert Stalnaker agree that facts about what a conscious experience is like to the one experiencing it cannot be deduced from knowing all the facts about the underlying physiology, but by contrast argue that such gaps of knowledge are also present in many other cases in nature, such as the distinction between water and H₂O.[95][11]

To explain why these two ways of knowing (i.e. third-person scientific observation and first-person introspection) yield such different understandings of consciousness, weak reductionists often invoke the phenomenal concepts strategy, which argues the difference stems from our inaccurate phenomenal concepts (i.e., how we think about consciousness), not from the nature of consciousness itself.[96][97] By this view, the hard problem of consciousness stems from a dualism of concepts, not from a dualism of properties or substances.[43]

Some consciousness researchers have argued that the hard problem is a cultural artifact, unique to contemporary Western Culture. This is similar to Type-B Materialism, but it makes the further claim that the psychological facts that cause us to intuit the hard problem are not innate, but culturally conditioned. Notable researchers who hold this view include Anna Wierzbicka,[98] Hakwan Lau and Matthias Michel.[99]

Wierzbicka (who is a linguist) argues that the vocabulary used by consciousness researchers (including words like experience and consciousness) are not universally translatable, and are "parochially English."[98] Weirzbicka calls David Chalmers out by name for using these words, arguing that if philosophers "were to use panhuman concepts expressed in crosstranslatable words" (such as know, think, or feel) then the hard problem would dissolve.[98] David Chalmers has responded to these criticisms by saying that he will not "apologize for using technical terms in an academic article . . . they play a key role in efficient communication in every discipline, including Wierzbicka’s".[91]

Dualism views consciousness as either a non-physical substance separate from the brain or a non-physical property of the physical brain.[100] Dualism is the view that the mind is irreducible to the physical body.[100] There are multiple dualist accounts of the causal relationship between the mental and the physical, of which interactionism and epiphenomenalism are the most common today. Interactionism posits that the mental and physical causally impact one another, and is associated with the thought of René Descartes (1596–1650).[59] Epiphenomalism holds the mental is causally dependent on the physical, but does not in turn causally impact it.[59]

In contemporary philosophy, interactionism has been defended by philosophers including Martine Nida-Rümelin,[101] while epiphenomenalism has been defended by philosophers including Frank Jackson[102][103] (although Jackson later changed his stance to physicalism).[104] Chalmers has also defended versions of both positions as plausible.[59] Traditional dualists such as Descartes believed the mental and the physical to be two separate substances, or fundamental types of entities (hence "substance dualism"); some more recent dualists, however, accept only one substance, the physical, but state it has both mental and physical properties (hence "property dualism").[100]

Meanwhile, panpsychism and neutral monism, broadly speaking, view consciousness as intrinsic to matter.[59] In its most basic form, panpsychism holds that all physical entities have minds (though its proponents take more qualified positions),[105] while neutral monism, in at least some variations, holds that entities are composed of a substance with mental and physical aspects—and is thus sometimes described as a type of panpsychism.[106]

Forms of panpsychism and neutral monism were defended in the early twentieth century by the psychologist William James,[107][108][note 2] the philosopher Alfred North Whitehead,[108] the physicist Arthur Eddington,[109][110] and the philosopher Bertrand Russell,[105][106] and interest in these views has been revived in recent decades by philosophers including Thomas Nagel,[108] Galen Strawson,[108][111] and David Chalmers.[105] Chalmers describes his overall view as "naturalistic dualism",[3] but he says panpsychism is in a sense a form of physicalism,[59] as does Strawson.[111] Proponents of panpsychism argue it solves the hard problem of consciousness parsimoniously by making consciousness a fundamental feature of reality.[43][112]

A traditional solution to the hard problem is idealism, according to which consciousness is fundamental and not simply an emergent property of matter. It is claimed that this avoids the hard problem entirely.[113] Objective idealism and cosmopsychism consider mind or consciousness to be the fundamental substance of the universe. Proponents claim that this approach is immune to both the hard problem of consciousness and the combination problem that affects panpsychism.[114][115][116]

From an idealist perspective, matter is a representation or image of mental processes. Supporters suggest that this avoids the problems associated with the materialist view of mind as an emergent property of a physical brain.[117] Critics point out that you then have a decombination problem, in terms of explaining individual subjective experience. In response, Bernardo Kastrup claims that nature hints at a mechanism for this in the condition Dissociative identity disorder (previously known as Multiple Personality Disorder).[118] Kastrup proposes dissociation as an example from nature showing that multiple minds with their own individual subjective experience could develop within a single universal mind.

Cognitive psychologist Donald D. Hoffman uses a mathematical model based around conscious agents, within a fundamentally conscious universe, to support conscious realism as a description of nature—one that falls within the objective idealism approaches to the hard problem: "The objective world, i.e., the world whose existence does not depend on the perceptions of a particular conscious agent, consists entirely of conscious agents."[119]

David Chalmers calls this form of idealism one of "the handful of promising approaches to the mind–body problem."[120]

New mysterianism, most significantly associated with the philosopher Colin McGinn, proposes that the human mind, in its current form, will not be able to explain consciousness.[121][10] McGinn draws on Noam Chomsky's distinction between problems, which are in principle solvable, and mysteries, which human cognitive faculties are unequipped to ever understand, and places the mind–body problem in the latter category.[121] His position is that a naturalistic explanation does exist but that the human mind is cognitively closed to it due to its limited range of intellectual abilities.[121] He cites Jerry Fodor's concept of the modularity of mind in support of cognitive closure.[121]

While in McGinn's strong form, new mysterianism states that the relationship between consciousness and the material world can never be understood by the human mind, there are also weaker forms that argue it cannot be understood within existing paradigms but that advances in science or philosophy may open the way to other solutions (see above).[43] The ideas of Thomas Nagel and Joseph Levine fall into the second category.[43] The cognitive psychologist Steven Pinker has also endorsed this weaker version of the view, summarizing it as follows:[30]

And then there is the theory put forward by philosopher Colin McGinn that our vertigo when pondering the Hard Problem is itself a quirk of our brains. The brain is a product of evolution, and just as animal brains have their limitations, we have ours. Our brains can't hold a hundred numbers in memory, can't visualize seven-dimensional space and perhaps can't intuitively grasp why neural information processing observed from the outside should give rise to subjective experience on the inside. This is where I place my bet, though I admit that the theory could be demolished when an unborn genius—a Darwin or Einstein of consciousness—comes up with a flabbergasting new idea that suddenly makes it all clear to us.

Since 1990, researchers including the molecular biologist Francis Crick and the neuroscientist Christof Koch have made significant progress toward identifying which neurobiological events occur concurrently to the experience of subjective consciousness.[123] These postulated events are referred to as neural correlates of consciousness or NCCs. However, this research arguably addresses the question of which neurobiological mechanisms are linked to consciousness but not the question of why they should give rise to consciousness at all, the latter being the hard problem of consciousness as Chalmers formulated it. In "On the Search for the Neural Correlate of Consciousness", Chalmers said he is confident that, granting the principle that something such as what he terms global availability can be used as an indicator of consciousness, the neural correlates will be discovered "in a century or two".[124] Nevertheless, he stated regarding their relationship to the hard problem of consciousness:

One can always ask why these processes of availability should give rise to consciousness in the first place. As yet we cannot explain why they do so, and it may well be that full details about the processes of availability will still fail to answer this question. Certainly, nothing in the standard methodology I have outlined answers the question; that methodology assumes a relation between availability and consciousness, and therefore does nothing to explain it. [...] So the hard problem remains. But who knows: Somewhere along the line we may be led to the relevant insights that show why the link is there, and the hard problem may then be solved.[124]

The neuroscientist and Nobel laureate Eric Kandel wrote that locating the NCCs would not solve the hard problem, but rather one of the so-called easy problems to which the hard problem is contrasted.[125] Kandel went on to note Crick and Koch's suggestion that once the binding problem—understanding what accounts for the unity of experience—is solved, it will be possible to solve the hard problem empirically.[125] However, neuroscientist Anil Seth argued that emphasis on the so-called hard problem is a distraction from what he calls the "real problem": understanding the neurobiology underlying consciousness, namely the neural correlates of various conscious processes.[21] This more modest goal is the focus of most scientists working on consciousness.[125] Psychologist Susan Blackmore believes, by contrast, that the search for the neural correlates of consciousness is futile and itself predicated on an erroneous belief in the hard problem of consciousness.[126]

Integrated information theory (IIT), developed by the neuroscientist and psychiatrist Giulio Tononi in 2004 and more recently also advocated by Koch, is one of the most discussed models of consciousness in neuroscience and elsewhere.[127][128] The theory proposes an identity between consciousness and integrated information, with the latter item (denoted as Φ) defined mathematically and thus in principle measurable.[128][129] The hard problem of consciousness, write Tononi and Koch, may indeed be intractable when working from matter to consciousness.[14] However, because IIT inverts this relationship and works from phenomenological axioms to matter, they say it could be able to solve the hard problem.[14] In this vein, proponents have said the theory goes beyond identifying human neural correlates and can be extrapolated to all physical systems. Tononi wrote (along with two colleagues):

While identifying the "neural correlates of consciousness" is undoubtedly important, it is hard to see how it could ever lead to a satisfactory explanation of what consciousness is and how it comes about. As will be illustrated below, IIT offers a way to analyze systems of mechanisms to determine if they are properly structured to give rise to consciousness, how much of it, and of which kind.[130]

As part of a broader critique of IIT, Michael Cerullo suggested that the theory's proposed explanation is in fact for what he dubs (following Scott Aaronson) the "Pretty Hard Problem" of methodically inferring which physical systems are conscious—but would not solve Chalmers' hard problem.[128] "Even if IIT is correct," he argues, "it does not explain why integrated information generates (or is) consciousness."[128] Chalmers agrees that IIT, if correct, would solve the "Pretty Hard Problem" rather than the hard problem.[131]

Global workspace theory (GWT) is a cognitive architecture and theory of consciousness proposed by the cognitive psychologist Bernard Baars in 1988.[132] Baars explains the theory with the metaphor of a theater, with conscious processes represented by an illuminated stage.[132] This theater integrates inputs from a variety of unconscious and otherwise autonomous networks in the brain and then broadcasts them to unconscious networks (represented in the metaphor by a broad, unlit "audience").[132] The theory has since been expanded upon by other scientists including cognitive neuroscientist Stanislas Dehaene.[133]

In his original paper outlining the hard problem of consciousness, Chalmers discussed GWT as a theory that only targets one of the "easy problems" of consciousness.[3] In particular, he said GWT provided a promising account of how information in the brain could become globally accessible, but argued that "now the question arises in a different form: why should global accessibility give rise to conscious experience? As always, this bridging question is unanswered."[3] J. W. Dalton similarly criticized GWT on the grounds that it provides, at best, an account of the cognitive function of consciousness, and fails to explain its experiential aspect.[134] By contrast, A. C. Elitzur argued: "While [GWT] does not address the 'hard problem', namely, the very nature of consciousness, it constrains any theory that attempts to do so and provides important insights into the relation between consciousness and cognition."[135]

For his part, Baars writes (along with two colleagues) that there is no hard problem of explaining qualia over and above the problem of explaining causal functions, because qualia are entailed by neural activity and themselves causal.[20] Dehaene, in his 2014 book Consciousness and the Brain, rejected the concept of qualia and argued that Chalmers' "easy problems" of consciousness are actually the hard problems.[19] He further stated that the "hard problem" is based only upon ill-defined intuitions that are continually shifting as understanding evolves:[19]

Once our intuitions are educated by cognitive neuroscience and computer simulations, Chalmers' hard problem will evaporate. The hypothetical concept of qualia, pure mental experience, detached from any information-processing role, will be viewed as a peculiar idea of the prescientific era, much like vitalism... [Just as science dispatched vitalism] the science of consciousness will keep eating away at the hard problem of consciousness until it vanishes.