What Is the Explanatory Gap?

Direct Answer

The explanatory gap is the apparent failure of physical and functional explanations to account for subjective experience. We can describe in exquisite detail the neural processes that occur when a person sees red, feels pain, or tastes coffee — which cells fire, which pathways activate, which computations are performed. Yet it seems that no amount of such description explains why the experience feels the way it does, or indeed why there is any experience at all. The gap is between the third-person story science tells about the brain and the first-person character of what it is like to be a subject having experiences.

The term was coined by the philosopher Joseph Levine in his 1983 paper "Materialism and Qualia: The Explanatory Gap." Levine's point was epistemic rather than metaphysical: even if materialism about the mind is true, there is a gap in our explanations — we cannot see how the facts about qualia follow from the facts about brain states. Explaining pain by identifying it with C-fiber stimulation, in the classic example, does not explain the painfulness of pain. The identity might be true, but it does not yield understanding, and that is what "explanatory gap" names: a deficit in intelligibility, not merely in knowledge.

The explanatory gap is closely related to, but distinct from, David Chalmers' hard problem of consciousness. The hard problem asks why physical processing gives rise to experience at all — a question about the metaphysics of consciousness. The explanatory gap asks whether physical science can make experience intelligible — a question about explanation. They are two sides of the same coin: if the gap is permanent, the hard problem cannot be dissolved; if it can be closed, the hard problem may never have been a genuine problem.

Different positions react differently. Physicalists like Daniel Dennett argue that the gap is an artifact of our current concepts — a matter of imaginative limitation that will close as neuroscience matures, just as the apparent gap between chemistry and biology closed. Dualists argue that the gap reflects a real metaphysical divide between mind and matter, and that no amount of science will bridge it. Panpsychists accept the gap but relocate consciousness to the fundamental level of reality, where no emergence needs explaining. Illusionists deny that there is any phenomenal experience for the gap to be about: if consciousness is an illusion, the gap dissolves along with it.

Historical Context

The explanatory gap is the modern descendant of a very old worry. Descartes argued that mind and matter are so different in kind that no physical explanation could account for thought — the ancestor of the gap, though Descartes drew a metaphysical conclusion where Levine drew an epistemic one. Hume pressed a related point about causation: we never observe the "necessity" that connects cause and effect, so our explanations rest on habit rather than insight — a skeptical note that later philosophers heard echoing through the gap.

In the twentieth century, the logical positivists — including A. J. Ayer and Bertrand Russell — debated whether talk of experience could be translated into physical talk, and Russell himself suggested that physics describes only the structure of matter, leaving its intrinsic nature — which he suspected was experiential — unknown. William James and Henri Bergson had already insisted that the stream of lived experience resists capture in the vocabulary of natural science. The contemporary debate crystallized in the 1970s and 1980s, with Thomas Nagel's "What Is It Like to Be a Bat?" (1974), Frank Jackson's knowledge argument, and Levine's coinage of the term itself.

Philosophical Significance

The explanatory gap matters because it determines what we can expect from a science of consciousness. If the gap is permanent, then the complete physical description of the brain will still leave something unexplained, and a purely physicalist worldview will be incomplete in a way that matters. If the gap can be closed, then consciousness research has a clear trajectory: better science will eventually make experience intelligible in physical terms, and the feeling of mystery will be revealed as an artifact of ignorance.

The gap also structures the contemporary options in philosophy of mind. It is the main motivation for property dualism: since physical explanation leaves experience out, experience must be a further, irreducibly mental feature of reality. It motivates panpsychism for the same reason. It motivates illusionism by inversion: since the gap cannot be closed, perhaps the thing it is about — phenomenal consciousness — does not exist. And it motivates a distinctive physicalist response: the "phenomenal concepts" strategy, which holds that the gap arises because our concepts of experience are special — we think about experience from the first-person perspective — even though the experience itself is fully physical. Whether the gap is an illusion of concepts or a genuine feature of reality remains the deepest unresolved question in the field.

Examples & Thought Experiments

The classic illustration is Mary, the color scientist. Mary has lived her entire life in a black-and-white room and has learned every physical fact about color vision — the wavelengths, the neural pathways, the computations. When she steps outside and sees red for the first time, she learns something new: what it is like to see red. If all the physical facts were already in her possession, and she still learned something, then the physical facts do not exhaust the facts about experience — the explanatory gap is real.

The philosophical zombie makes the point modally. If a physical duplicate of a conscious being without experience is conceivable, then the physical facts do not entail the phenomenal facts — and, on the standard reading, no physical explanation can bridge the gap. The inverted spectrum thought experiment makes the point about qualia: it seems possible that your experience of red is qualitatively like my experience of green, even though our behavior and neural responses are identical. If that is possible, then the qualitative character of experience — the very thing the gap concerns — is not captured by functional or physical description.

A humbler illustration is available to everyone: try to explain to a congenitally blind person what it is like to see. You can describe the physics of light, the anatomy of the eye, the brain's computations — but you cannot convey the experience itself. That failure is the explanatory gap in miniature.

Related Concepts

• Consciousness and qualia
• The hard problem of consciousness
• Philosophical zombies
• Dualism, panpsychism, and illusionism
• Functionalism
• Mind and scientific method

Further Learning

• Stanford Encyclopedia of Philosophy, Consciousness
• Internet Encyclopedia of Philosophy, Qualia
• Internet Encyclopedia of Philosophy, The Hard Problem of Consciousness
• Joseph Levine, "Materialism and Qualia: The Explanatory Gap," Pacific Philosophical Quarterly 64 (1983)