In philosophy of mind, the hard problem of consciousness (or simply the hard problem) is to explain how and why organisms have qualia, phenomenal consciousness, or subjective experience. It is contrasted with the "easy problems" of explaining why and how physical systems give rise to the ability to discriminate, to integrate information, and to perform behavioural functions such as watching, listening, speaking (including generating an utterance that appears to refer to personal behaviour or belief), and so forth. The easy problems are amenable to functional explanation—that is, explanations that are mechanistic or behavioural—since each physical system can be explained purely by reference to the "structure and dynamics" that underpin the phenomenon.
Proponents of the hard problem hold that it is categorically different from the easy problems since no mechanistic or behavioural explanation could explain the character of an experience, not even in principle. Even after all the relevant functional facts are explicated, they argue, there will still remain a further question: "Why is the performance of these functions accompanied by experience?" To bolster their case, proponents of the hard problem frequently turn to various philosophical thought experiments, involving philosophical zombies, or inverted qualia, or the ineffability of colour experiences, or the unknowability of foreign states of consciousness, such as the experience of being a bat.
The terms "hard problem" and "easy problems" were coined by the philosopher David Chalmers in a 1994 talk given at The Science of Consciousness conference held in Tucson, Arizona.Problem of Consciousness (Tuscan 1994) The following year, the main talking points of Chalmers' talk were published in The Journal of Consciousness Studies. The publication gained significant attention from consciousness researchers and became the subject of a special volume of the journal,JCS vol. 4, pp. 3-46, 1997 which was later published as a book. In 1996, Chalmers published The Conscious Mind, a book-length treatment of the hard problem, in which he elaborated on his core arguments and responded to counterarguments. His use of the word easy is "tongue-in-cheek". As the cognitive psychologist 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."
The existence of the hard problem is disputed. It has been accepted by some philosophers of mind such as Joseph Levine, Colin McGinn, and Ned Block and cognitive neuroscientists such as Francisco Varela, Giulio Tononi, and Christof Koch. On the other hand, its existence is rejected by other philosophers of mind, such as Daniel Dennett, Massimo Pigliucci, Thomas Metzinger, Patricia Churchland, and Keith Frankish, and by cognitive neuroscientists such as Stanislas Dehaene, Bernard Baars, Anil Seth, and Antonio Damasio. Clinical neurologist and sceptic Steven Novella has dismissed it as "the hard non-problem". According to a 2020 PhilPapers survey, a majority (62.4%) of the philosophers surveyed said they believed that the hard problem is a genuine problem, while 29.7% said that it does not exist.
There are a number of other potential philosophical problems that are related to the Hard Problem. Ned Block believes that there exists a "Harder Problem of Consciousness", due to the possibility of different physical and functional neurological systems potentially having phenomenal overlap. Another potential philosophical problem which is closely related to Benj Hellie's vertiginous question, dubbed "The Even Harder Problem of Consciousness", refers to why a given individual has their own particular personal identity, as opposed to existing as someone else. Finally, proposals in the 2020s have been made to reconcile views from neuroscience and the philosophy of mind from a representationalist perspective. This holds in particular for a cognitively inspired form of representationalism, which is presumed to offer a favourable perspective for bridging gaps between different views in cognitive neuroscience and the philosophy of mind, relating to concepts such as intentionality, emergence, consciousness, and qualia.
Overview Cognitive scientist David Chalmers first formulated the hard problem in his 1995 paper, "Facing up to the problem of consciousness", and expanded upon it in The Conscious Mind (1996). His works provoked comment. Some, such as philosopher David Lewis and Steven Pinker, have praised Chalmers for his argumentative rigour and "impeccable clarity". In 2018, Pinker said: "In the end I still think that the hard problem is a meaningful conceptual problem, but agree with Dennett that it is not a meaningful scientific problem. No one will ever get a grant to study whether you are a zombie or whether the same Captain Kirk walks on the deck of the Enterprise and the surface of Zakdorn. And I agree with several other philosophers that it may be futile to hope for a solution at all, precisely because it is a conceptual problem, or, more accurately, a problem with our concepts." Daniel Dennett and Patricia Churchland, among others, believe that the hard problem is best seen as a collection of easy problems that will be solved through further analysis of the brain and behaviour. Consciousness is an ambiguous term. It can be used to mean self consciousness, awareness, the state of being awake, and so on. Chalmers uses Thomas Nagel's definition of consciousness as "the feeling of what it is like to be something". In this sense, consciousness is synonymous with experience.
Chalmers' formulation The problems of consciousness, Chalmers argues, are of two kinds: the easy problems and the hard problem.
Easy problems The easy problems are amenable to reductive enquiry. 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 being 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). 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 analysed through "structures and functions".
Hard problem The hard problem, in contrast, is the problem of why and how those processes are accompanied by experience. 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. 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.
How the easy and hard problems are related 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 problem. 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. 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. Consciousness, then, must not be purely physical.
Implications for physicalism
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. For example, the rings of Saturn are a physical thing because they are nothing more than a complex arrangement of a large number of subatomic particles interacting in a certain way. According to physicalism, everything, including consciousness, can be explained by appeal to its microphysical constituents. Chalmers's hard problem presents a counterexample to this view, since it suggests that consciousness cannot be reductively explained by appealing to its physical constituents. Chalmers argues that if the hard problem is a genuine obstacle, it follows that physicalism must be false. Conversely, if physicalism is true, the hard problem cannot be a real problem.
Although Chalmers rejects physicalism, he is still a naturalist. He proposes that consciousness is a fundamental feature of the natural world, not something supernatural, and seeks a nonreductive explanation within a naturalistic framework.Chalmers, "Facing Up to the Problem of Consciousness", pp. 209–10.