The hard problem of consciousness

What it is

Explain everything a brain does — how it discriminates light from dark, integrates information, directs attention, controls behaviour, produces speech about its own states — and one question is left standing.

Why is any of it experienced? Why is there something it is like to see red, rather than the whole business proceeding in the dark, with all the same processing and all the same behaviour and nobody home?

David Chalmers named these the easy problems and the hard problem. The easy problems are only easy by comparison: they are matters of explaining functions, and we know in principle what a solution looks like even where we are nowhere near one. The hard problem is different in kind. Once every function is explained, the question of why the functioning is accompanied by experience appears untouched.

The gap has a shape worth being precise about. It is not that we lack the neuroscience. It is that it is unclear what kind of fact about a physical system could explain why it feels like anything, in the way that facts about H₂O molecules explain why water is wet.

Where it came from

The name is recent; the problem is not.

Leibniz asked in 1714 to imagine a thinking machine enlarged to the size of a mill, so you could walk inside it. You would find parts pushing on parts, and nothing that could be a perception. Three centuries later that is still the intuition being argued about.

The modern sequence runs through three papers. Thomas Nagel's "What Is It Like to Be a Bat?" (1974) argued that no amount of objective knowledge about echolocation would tell you what bat experience is like from the inside, and gave the whole field its working definition of consciousness. Frank Jackson's knowledge argument (1982) — the story of Mary, a scientist who knows every physical fact about colour vision but has only seen black and white, and who seems to learn something new on leaving the room — pressed the same point. Joseph Levine (1983) named the explanatory gap.

Chalmers put the label on it in 1994 at the Tucson conference and in the 1995 paper "Facing Up to the Problem of Consciousness". The phrase spread far beyond philosophy almost immediately.

How it is used

Within philosophy it is the central organising problem of the field, and positions are largely defined by their response: solve it, dissolve it, or accept that it forces a change in what we take the fundamental furniture of reality to be.

Outside philosophy it has become one of the most borrowed ideas in modern spiritual writing, usually as a lever: science cannot explain consciousness, therefore — and then whatever the author wished to establish. Therefore the soul. Therefore survival of death. Therefore the universe is aware. Therefore this particular practice works.

Two things are worth saying about that, and they cut both ways.

The hard problem is real, and taken seriously by serious people. Dismissing it as mysticism is not defensible; a large body of technical philosophical work says otherwise.

But it is a problem, not a result. It does not support any specific positive claim. Nagel is an atheist. Chalmers is a naturalist who has consistently said his view requires no supernatural anything. An unexplained phenomenon licenses further inquiry — it does not license filling the gap with whichever answer you already preferred. That move has a name, and the name is not flattering.

What we can and cannot say

We can say the problem is genuine, widely acknowledged and unsolved, and that no current theory commands consensus. Global workspace theory, integrated information theory, higher-order theories and predictive processing accounts all have serious defenders, and each is accused by critics of explaining the easy problems while renaming the hard one.

We can say there is a substantial school that denies the problem exists. Illusionists — Daniel Dennett, Keith Frankish and others — argue that the sense of an unexplained residue is itself something to be explained rather than taken at face value: not that consciousness is unreal, but that its apparent irreducibility is a trick the system plays on itself. Most people find this deeply counterintuitive on first encounter, which illusionists say is exactly what their theory predicts. It is a live minority position, not a dodge.

We cannot say the problem proves physicalism false. We cannot say it will never be solved. And we certainly cannot say it supports any particular metaphysical or spiritual conclusion — not survival after death, not a universal mind, not the efficacy of any practice.

The honest summary is uncomfortable and worth sitting with: the most familiar fact in your life, the one thing you have direct access to and cannot doubt, is the thing our best account of the world has the most trouble accommodating. That is genuinely strange. Strangeness is not an argument, but it is a reasonable place to start asking questions, and it is why this section exists before any other on the site.

Further reading

• Thomas Nagel, "What Is It Like to Be a Bat?" (1974) — short, famous, and worth reading in the original rather than in summary.
• David Chalmers, "Facing Up to the Problem of Consciousness" (1995) — the paper that named it.
• Keith Frankish, "Illusionism as a Theory of Consciousness" (2016) — the strongest statement of the opposing view.