Explanatory Gap Argument

Overview and Historical Context

The explanatory gap argument is a central argument in the philosophy of mind highlighting a seeming disconnect between physical descriptions of the brain and the phenomenal character of conscious experience. It focuses on what are often called qualia: the subjective, “what-it-is-like” aspects of experience, such as the redness of red or the painfulness of pain.

The term “explanatory gap” was introduced by the philosopher Joseph Levine in a 1983 article, where he argued that even if physicalism (the doctrine that everything is ultimately physical) is true, there remains a striking gap in how physical facts could explain phenomenal consciousness. This argument builds on earlier concerns raised by figures such as Thomas Nagel in “What Is It Like to Be a Bat?” (1974), which emphasized the irreducibly subjective nature of experience.

The explanatory gap argument has become a major touchstone in debates about consciousness, reductionism, and the mind–body problem, and it is closely related to the so‑called “hard problem of consciousness” later articulated by David Chalmers.

Core Argument and Key Concepts

At the heart of the explanatory gap argument is a contrast between two kinds of description:

• Physical / functional descriptions: Objective, third‑person accounts of brain states, neural processes, and functional organization.
• Phenomenal descriptions: Subjective, first‑person accounts of how experiences feel from the inside.

Physicalism holds that facts about consciousness are wholly determined by physical facts. A strong version says that once all physical facts are fixed, all facts about qualia are automatically fixed as well. The explanatory gap argument challenges whether this claim can be made intelligible.

Levine’s formulation does not necessarily deny physicalism’s truth, but stresses a failure of explanation. Even if we knew everything about the brain at the microphysical level, it seems we could still sensibly ask: Why do these processes feel like anything at all, and why do they feel this particular way rather than another? The inability to see an a priori or conceptual link between brain facts and phenomenal facts is the gap.

A common way to illustrate the gap uses conceivability considerations (although Levine himself is cautious about this). Philosophers note that we can seemingly conceive of:

• A world physically identical to ours but without consciousness (sometimes called a world of “philosophical zombies”), or
• A being with all our physical and functional properties but lacking phenomenal experience.

If such scenarios are coherent, it suggests that physical information alone does not entail or make transparent the existence and character of qualia. Thus, even ideal physical explanations appear incomplete with respect to consciousness.

The gap is therefore explanatory rather than merely epistemic in a simple sense. It is not just that we do not yet know the details; the worry is that the very form of physical explanation—structural, relational, functional—seems ill‑suited to capture intrinsic, qualitative, first‑person aspects of experience.

Implications and Responses

The explanatory gap argument has significant implications for debates about the mind, especially concerning whether and how consciousness can be reduced to physical processes.

1. Anti‑reductionist and dualist reactions

Some philosophers take the explanatory gap to indicate that reductive physicalism is false or incomplete. They argue that:

• If physical facts do not a priori entail phenomenal facts, then mental properties may be distinct from physical properties.
• This supports forms of property dualism, where mental properties are non‑physical yet systematically correlated with physical states.
• Others advocate panpsychism or related views, according to which some form of experiential property is fundamental, precisely to avoid a gap between the physical and the phenomenal.

On these views, the gap is not a temporary limitation, but a sign that our metaphysics must accommodate consciousness as more than a derivative or purely physical phenomenon.

2. Physicalist strategies

Physicalists typically respond by accepting the appearance of a gap but offering different diagnoses of its significance:

• Ignorance hypotheses: Some argue that the gap reflects our current conceptual or scientific limitations. On this view, once neuroscience and cognitive science advance sufficiently—or once we develop new concepts—the gap may close. The gap is epistemic (about what we know and how we think), not ontological (about what exists).

• Type‑B materialism: Many physicalists concede that there is no a priori deduction of phenomenal truths from physical truths, but maintain that there is still an a posteriori identity between them (similar to water = H₂O). The explanatory gap is then akin to our initial puzzlement about how heat could “just be” molecular motion: real but ultimately surmountable.

• Phenomenal concept strategies: Some propose that the gap arises from the special nature of our concepts of experience. We possess phenomenal concepts that refer to experiences in a way that makes them seem independent of physical properties. The gap then reflects features of our conceptual scheme, not the underlying reality.

• Deflationary views of qualia: Other responses challenge the coherence of “qualia” as traditionally conceived. If qualia are redescribed in purely functional, relational, or representational terms, the apparent gap might dissolve, or at least be reframed as a dispute about language and classification rather than about ontology.

3. Relation to the hard problem of consciousness

The explanatory gap is closely aligned with the hard problem of consciousness, which asks: why and how do physical processes give rise to subjective experience at all? Many take the explanatory gap argument as a formal articulation of the hard problem’s core intuition. Critics, however, argue that the hard problem and the gap may overstate what explanation in science should accomplish, or rely on contentious assumptions about what an adequate explanation must look like.

Overall, the explanatory gap argument remains controversial. It has not produced consensus about the metaphysics of mind, but it has sharpened the central questions: what would count as a satisfactory explanation of consciousness, and can such an explanation be given entirely in physical terms? The persistence of divergent answers underscores the argument’s continuing importance in contemporary philosophy of mind.