The explanatory gap is a term coined by Joseph Levine and used in the philosophy of mind to refer to the absence of a generally acceptable physical explanation for mental phenomena like consciousness, self-awareness or the experiencing of qualia. Essentially, the claim is that phenomenal consciousness, the actual experiencing of qualia such as colours or pain, has not been explained in physical terms.

Although we can describe patterns of neuron (nerve cell) firing or neural pathways from the retina of the eye to the visual cortex of the brain and beyond, none of that has to do with the actual subjective experience that we each know first-hand, that is to say, what it actually feels like to see red or have pain, etc. This gap (or chasm, as some would prefer to call it) is what makes the 'hard problem' of understanding the relation of mind to body hard. It is an essential attack on materialism or physicalism in the mind-body discussion. Another aspect of the gap is that even if the experiencing of qualia is a physical state or results from one, the distinctive features of qualia cannot be described. What makes red ‘red’? What makes red different from, say, green? Here, the claim is that qualia are irreducible or indescribable.

There are three basic stances regarding the explanatory gap. Some philosophers say, "Gap? What gap?" They deny that an explanatory gap exists, claiming either that mental phenomena can, and indeed have been, explained in physical terms alone or that the gap is just an illusion (e.g., Daniel Dennett, Paul M. Churchland, Patricia Smith Churchland and Michael Tye). Others agree that the gap exists (i.e., that mental phenomena have not been adequately explained), but believe that a purely physical explanation will eventually be achieved, once the proper conceptual framework has been established (Thomas Nagel and John Searle). The third group insists not only that the gap exists, but also that no purely physical explanation is possible, because the stuff of mind is entirely different from the stuff of physics. These are the anti-materialists or dualism-interactionists (e.g., David Chalmers, Karl Popper and John C. Eccles).

The closing of the explanatory gap, or at least positive affirmation that the gap is illusory, is the nut of the problem for a materialist solution to the mind-body problem. For the anti-materialists, the need is to develop something more convincing than a prima facie argument to prove that a materialist explanation is impossible. Until one or the other occurs, the situation will remain in stalemate, with both sides simply believing what they are inclined to believe.