The explanatory gap is Joseph Levine’s name for a puzzle that survives even a true physicalism: granting that mental states are brain states, we still cannot see why a given brain state should feel the way it does. Levine introduced the term in “Materialism and Qualia: The Explanatory Gap” (1983). The move is subtler than Chalmers’s later hard problem and worth keeping distinct from it.
Levine’s Specific Move
Levine grants the physicalist that mental states might be brain states. The identity claim — pain = C-fiber firing, or whatever the right neural correlate turns out to be — might be true. The question Levine asks is not metaphysical but explanatory: even granting the identity, we cannot see why a brain state with that physical description should feel like that. The identity, if it holds, holds as a brute fact. We cannot derive the phenomenal character from the physical description.
This is the explanatory gap: a gap between the identity claim (metaphysical) and our ability to explain why the identity holds (epistemic). The physicalist might be right that only one kind of thing exists — the physical — and an explanatory gap would still stand between the third-person physical description and the first-person phenomenal character it identifies with.
Levine’s point is therefore not that physicalism is false. He explicitly stops short of that conclusion. His point: physicalism, even if true, leaves something unexplained — why the physical realizer feels the way it feels rather than some other way, or no way at all.
Distinguishing the Gap from the Hard Problem
Chalmers’s hard problem (1995) is often presented as continuous with Levine’s explanatory gap, but the two moves differ in ambition.
| Levine (1983) | Chalmers (1995) | |
|---|---|---|
| Type of gap | Epistemic | Argued toward ontological |
| Conclusion | Physicalism unexplained | Physicalism inadequate; property dualism |
| Argumentative use | Diagnostic of a residual mystery | Premise in a metaphysical argument |
| Stance toward physicalism | Withholds judgment | Rejects |
Levine treats the gap as a feature of our conceptual access; Chalmers treats the gap as evidence that phenomenal properties cannot be reduced to physical properties — and infers non-physical (or at least non-reductively-physical) phenomenal facts. A type-B physicalism accepts something close to Levine’s framing while rejecting Chalmers’s escalation. See The Hard Problem of Consciousness for the full Chalmers engagement.
The reason to keep the two distinct: Levine’s argument is much harder to dismiss. Granting the gap as an epistemic feature, while denying that it shows what Chalmers claims, is the physicalist’s middle position. Confusing the two moves makes that physicalism look as though it dismisses Levine, when in fact it accepts Levine and rejects only Chalmers’s inference from Levine.
The Phenomenal Concept Strategy
One major physicalist reply to Levine is the phenomenal concept strategy. Versions appear in Loar, Papineau, Balog, and others; Tye’s mature materialism retains acquaintance while rejecting the need for a proprietary class of phenomenal concepts.
The label covers several accounts: recognitional, demonstrative, quotational or constitutional, and direct-reference theories. Their common physicalist use is to explain how first-person and physical concepts can remain cognitively isolated while referring to one state. The book adopts a narrower mechanism: acquaintance with an occurring world-presenting state grounds recognitional capacities that description alone does not confer.
By undergoing an experience, a subject enters an acquaintance relation with occurring world-directed content and gains a recognitional capacity: a way of re-identifying the presented property across later occasions and, potentially, in other perceivers. No third-person description by itself confers that capacity, though descriptions can prepare and refine it. Acquaintance fixes a present reference; it does not reveal an infallible inner sample.
For knowers with our architecture, descriptive and acquaintance-grounded routes can therefore remain conceptually isolated while reaching one physical fact. Mary’s release supplies the canonical illustration. She already knows every physical fact about red; seeing red gives her an acquaintance-grounded way of representing and later recognizing a property she previously knew only through description. She gains a real cognitive capacity, not a new non-physical fact. See Mary's Room.
Where the Strategy Stops Short
The phenomenal concept strategy answers Levine in his own terms but leaves residual work. Two pieces stand out.
Why these concepts have this structure. A complete account would explain why phenomenal concepts are the way they are — why they require acquaintance, why they don’t reduce to functional descriptions. On an intentionalist development of the strategy, the answer comes from intentionalism plus embodiment: phenomenal concepts are recognitional capacities for representational contents that are themselves world-directed and physically realized. The concepts inherit their structure from the contents they represent. This runs deeper than the bare phenomenal-concept reply.
Why the gap feels metaphysical. Even after the epistemic reply is accepted, the feeling that more than an epistemic gap remains tends to persist. The intentionalist diagnosis traces this feeling to an artifact of the inner-theater picture: so long as phenomenal character gets implicitly conceived as an intrinsic inner property, the gap between description and acquaintance feels like a gap in nature. Drop the picture, recast phenomenal character as representational content, and the residual feeling has less to feed on. The gap becomes a fact about how the content is accessed, not about what kind of content it is. See The Inner Theater and Intentionalism.
My View
My deployment of the explanatory gap is threefold.
Accept the diagnosis. Yes, there is an epistemic gap between third-person physical description and first-person phenomenal acquaintance. Levine is right about this.
Use acquaintance and recognition. The gap is real but not ontologically loaded. Description and first-person recognition provide different routes to one physical fact; explaining the recognitional capacity does not confer it.
Diagnose the residual mystery as inner-theater inheritance. When the gap feels like more than an epistemic gap — when it feels like evidence for non-physical phenomenal properties — the inner-theater picture is doing that work in the background. Name the picture and drop it, and the residual feeling loses its force. The intuition that “something is still left out” trades on the picture; the picture is what gets left out, not a phenomenon.
I treat Levine’s gap as a useful diagnostic tool, a way of marking exactly what physicalism does and does not make available a priori, without treating the gap as a second feature of nature. The acquaintance-recognition account explains why the gap persists given the identity; it cannot establish the identity. Chapters 6 and 11 carry those burdens separately.
Related Concepts
- The Hard Problem of Consciousness — Chalmers’s escalation of Levine’s gap; my view accepts Levine, rejects Chalmers’s inference
- Qualia — qualia realism is one way of reading the gap as ontological; my view reads the gap epistemically instead
- Mary's Room — Jackson’s argument is the gap made vivid; same physicalist response (representational capacity gain, not non-physical fact discovery)
- Intentionalism — phenomenal character as representational content; the deeper story behind the phenomenal-concept reply
- Physicalism — the metaphysical commitment within which the gap is handled
- The Inner Theater — what makes the gap feel ontological when it isn’t
- Identity Theory — the position Levine’s argument originally targeted; my view inherits identity theory’s physicalist commitment while moving past type-identity