Levine’s Explanatory Gap

Philosopher Joseph Levine coined the term "explanatory gap" to name a precise failure: even if a conscious state is identical to some neural state, we cannot see why that neural state should feel the way it does, and further physical detail does not help (Levine, 1983). His signature move is to insist that this failure is epistemological rather than ontological—it marks a limit on our explanatory resources, not evidence that consciousness is nonphysical—which puts him at odds simultaneously with dualists who read the gap as a metaphysical finding and with materialists who claim to have closed it. He characterizes this position as a Kantian antinomy: excellent reasons to think phenomenal consciousness must be physical, since it has physical causes and effects, alongside excellent reasons to think it cannot be captured in physical terms (Levine, 2001). What Levine offers is therefore not a theory of consciousness but a rigorous account of why we do not have one.

In other words, Levine’s explanatory-gap argument holds that even if phenomenal consciousness is wholly dependent on—or even identical with—physical processes in the brain, a complete physical-functional description appears unable to explain why those processes have the particular subjective character they do. Introduced as an epistemological reformulation of Kripke’s challenge to mind-brain identity, the explanatory gap is deliberately weaker than an argument for dualism: the failure of intelligible reduction does not by itself prove that phenomenal and physical properties are metaphysically distinct (Levine, 1983, 2001). Levine’s later work, however, increasingly questions whether physicalism can satisfy this explanatory demand and has moved toward strong emergentism and a primitive relation of subjective experience.

From Kripke to the Explanatory Gap

Levine introduced the term “explanatory gap” in Materialism and Qualia: The Explanatory Gap as a response to Saul Kripke’s modal argument against psychophysical identity. Kripke had argued that the felt contingency of psychophysical identities cannot be explained away as it can for other theoretical identifications. He maintained that if pain were identical with some neural state—famously, C-fiber firing—the identity would be necessary if true, yet pain without that neural state appears conceivable, and had drawn a dualist conclusion (Kripke, 1980)

Levine thought the intuition behind Kripke’s argument was important but refused its metaphysical conclusion. Instead, it revealed an epistemic asymmetry: psychophysical identities, unlike paradigmatic scientific identities, leave something crucial unexplained (Kripke, 1980; Levine, 1983). The asymmetry is real but epistemic: what differs between the two cases is the explanatory adequacy of the identity statement, not its modal status. The resistance to materialism should not be dismissed as merely epistemology, he insisted, but neither does it license an ontological verdict.

Consider an identity such as heat = molecular motion. Once the relevant science is understood, molecular kinetic theory makes intelligible why heat behaves as it does—expands metals, flows from hot to cold, feels warm to a perceiver. The identity is fully explanatory: nothing crucial is left out. Now consider "pain is the firing of C-fibers." Even granting the identity, a question survives that has no analog in the heat case—why should that firing feel exactly this way rather than like some other way, or why should it feel like anything at all? The physical account fixes the causal and functional profile of pain while leaving its qualitative character unaccounted for (Levine, 1983). Thus, the phenomenal character appears as an additional brute fact. Levine’s point is not that the identity must therefore be false, but that its truth would not provide the explanatory satisfaction ordinarily associated with scientific reduction (Levine, 1983, 2001).

The gap thus belongs specifically to explanation. It is not the claim that we lack knowledge of some fact, as in the knowledge argument, nor that a metaphysical possibility has been established, as in the conceivability argument. It is the claim that a kind of intelligibility routinely available elsewhere in science is systematically unavailable for phenomenal consciousness.

Why Levine Remains a Materialist

The materialist side of the antinomy rests on mental causation. Conscious states have physical effects—we withdraw, we report, we act on what we feel—and if the physical world is causally closed, conscious states must be physical or else we are overwhelmed by systematic overdetermination (Levine, 2001). Levine takes this to be strong enough that dualism is not a live option, however unsatisfying the alternative.

He accordingly defends materialism against the modal argument, holding that the route from conceivability to metaphysical possibility rests on a questionable conflation of semantic with metaphysical issues (Levine, 1998; Chalmers, 1996). Zombies are imaginable; it does not follow that they are possible. His contrast between modal rationalism and what he terms modal autonomism marks the point—whether conceptual coherence tracks metaphysical possibility is itself a substantive question, and the anti-materialist argument presupposes an answer it has not demonstrated (Levine, 2018).

Why the Materialist Repairs Fail

Levine is equally unsparing toward strategies meant to dissolve the gap from the materialist side. Against the phenomenal concept strategy—on which the gap is an artifact of how our phenomenal concepts work rather than a feature of the world—he argues that the strategy is self-undermining. It requires that the peculiarity of phenomenal concepts be itself physically explicable, yet the feature that must be invoked, the intimate relation between a phenomenal concept and the experience it denotes, is precisely what stands in need of physical explanation (Levine, 2007).

Against representationalism—on which phenomenal character reduces to representational content—he argues the view has fatal flaws when joined to a materialist metaphysics. Representational content is itself naturalized in causal or teleofunctional terms, and those terms leave the same residue: one can specify what a state represents without specifying what it is like to be in it (Levine, 2003, 2018). Across his work, the pattern holds—every reductive strategy either helps itself to an unexplained phenomenal element or changes the subject.

Acquaintance and the Shape of an Adequate Account

Levine's later work sketches what an adequate account would require without claiming to provide one. He develops a conception of experience organized around acquaintance and appearance rather than representation: in having an experience one stands in relation to a qualitative character that is not merely represented but presented, and this presentational structure resists functional and representational analysis (Levine, 2018). His persistent claim is that materialists have underestimated or underdescribed what needs explaining, and that describing it accurately is what makes the gap look unbridgeable.

From Agnosticism Toward Strong Emergence

Levine’s position has evolved significantly. Purple Haze emphasized the tension between compelling reasons for materialism and the persistent inability of materialism to explain phenomenal experience, without treating the gap as a deductive refutation of physicalism (Levine, 2001). More recently, however, Levine has stated that he “strongly incline[s]” toward strong emergentism: phenomenal conscious states are not realized by or metaphysically supervenient on underlying physical-functional states but arise according to basic psychophysical laws (Levine, 2019).

On this later view, a physically identical zombie world is metaphysically possible, though not nomologically possible given the psychophysical laws of our world. Levine combines this with an acquaintance-based picture of consciousness in which subjects stand in a primitive experiential relation to a phenomenally presented world. Physical and computational mechanisms can explain much of the structure of experience—for example, similarity relations within color space—without explaining why a particular physical structure is accompanied by redness rather than greenness, or by any phenomenality whatsoever. Those qualitative correspondences may ultimately be brute psychophysical facts (Levine, 2019, 2021).

Explanatory Significance, Open Questions, Assessment

Levine's influence is disproportionate to the modesty of his positive claims. The explanatory gap is a fixed term of art, an acknowledged precursor to Chalmers's hard problem, and the standing challenge materialist theories must meet. His refusal to convert it into an argument for dualism makes him an unusually credible critic of materialism from within. The explanatory gap has enduring force because it isolates a problem that survives detailed neuroscience: neural correlates can become increasingly exact while the question of why those states possess phenomenal character remains conceptually separable and forever in the future. Its neutrality is also a strength—the original argument does not simply infer dualism from ignorance.

Levine’s mature emergentism sharpens the stakes. If phenomenal consciousness is genuinely basic relative to the physical, the explanatory gap reflects an ontological discontinuity rather than merely our present neuroscience knowledge. This creates corresponding problems of mental causation, epiphenomenalism, and explaining how acquaintance can influence physical cognition—difficulties Levine explicitly recognizes. The lasting contribution of the explanatory gap is therefore less a solution than a stringent criterion for one: a theory of consciousness must explain not merely which physical processes correlate with experience, but why and how those processes constitute, necessitate, or otherwise yield this astonishing subjective experience itself.

Critics respond that Levine may demand a stronger form of intelligibility from consciousness than science can legitimately provide, or that some fundamental identities and laws simply terminate explanation. Physicalists additionally argue that phenomenal-concept theories can explain the apparent gap without requiring an ontological divide (Levine, 2018; Melnyk, 2002). Type-B materialists hold that his own diagnosis removes the gap's sting: an epistemic gap with no ontological consequence is what one should expect given how phenomenal concepts function. Illusionists argue the gap arises only because the explanandum has been described in question-begging terms. Others find the position unstable—if materialism is true and the gap merely epistemic, it is puzzling why the obstacle should be permanent, while if it is permanent, that permanence invites an ontological explanation. Levine's answer is the antinomy: he accepts the discomfort as the honest response to the evidence rather than a defect to be argued away.

References

Chalmers, D. J. (1995). Facing up to the problem of consciousness. Journal of Consciousness Studies, 2(3), 200–219.

Chalmers, D. J. (1996). The Conscious Mind: In Search of a Fundamental Theory. Oxford University Press.

Kripke, S. A. (1980). Naming and Necessity. Harvard University Press.

Levine, J. (1983). Materialism and qualia: The explanatory gap. Pacific Philosophical Quarterly, 64, 354–361.

Levine, J. (1993). On leaving out what it's like. In M. Davies & G. Humphreys (Eds.), Consciousness: Psychological and Philosophical Essays (pp. 121–136). Blackwell.

Levine, J. (1998). Conceivability and the metaphysics of mind. Noûs, 32(4), 449–480.

Levine, J. (2001). Purple Haze: The Puzzle of Consciousness. Oxford University Press.

Levine, J. (2003). Experience and representation. In Q. Smith & A. Jokic (Eds.), Consciousness: New Philosophical Perspectives. Oxford University Press.

Levine, J. (2007). Phenomenal concepts and the materialist constraint. In T. Alter & S. Walter (Eds.), Phenomenal Concepts and Phenomenal Knowledge: New Essays on Consciousness and Physicalism. Oxford University Press.

Levine, J. (2018). Quality and Content: Essays on Consciousness, Representation, and Modality. Oxford University Press.

Levine, J. (2019a). On the meta-problem. Journal of Consciousness Studies, 26(9–10), 148–159.

Levine, J. (2019b). Consciousness is acquaintance and acquaintance is consciousness. In J. Knowles & T. Raleigh (Eds.), Acquaintance: New Essays. Oxford University Press.

Levine, J. (2021). The metaphysics and epistemology of acquaintance. ProtoSociology, 38, 15–34.

Melnyk, A. (2002). Review of Joseph Levine, Purple Haze: The Puzzle of Consciousness. Philosophy and Phenomenological Research.