The explanatory gap is a term introduced by philosopher Joseph Levine for the difficulty that physicalist theories of mind have in explaining how physical properties give rise to the way things feel when they are experienced. In the 1983 paper in which he first used the term, he used as an example the sentence, "Pain is the firing of C fibers", pointing out that while it might be valid in a physiological sense, it does not help us to understand how pain feels.
The explanatory gap has vexed and intrigued philosophers and AI researchers alike for decades and caused considerable debate. Bridging this gap (that is, finding a satisfying mechanistic explanation for experience and qualia) is known as "the hard problem".
To take an example of a phenomenon in which there is no gap, imagine a modern computer: as marvelous as these devices are, their behavior can be fully explained by their circuitry, and vice versa. By contrast, it is thought by many mind-body dualists (e.g. René Descartes, David Chalmers) that subjective conscious experience constitutes a separate effect that demands another cause, a cause that is either outside the physical world (dualism) or due to an as yet unknown physical phenomenon (see for instance Quantum mind, Indirect realism).
The nature of the explanatory gap has been the subject of some debate. For example, some consider it to simply be a limit on our current explanatory ability. They argue that future findings in neuroscience or future work from philosophers could close the gap. However, others have taken a stronger position and argued that the gap is a definite limit on our cognitive abilities as humans—no amount of further information will allow us to close it. There has also been no consensus regarding what metaphysical conclusions the existence of the gap provides. Those wishing to use its existence to support dualism have often taken the position that an epistemic gap—particularly if it is a definite limit on our cognitive abilities—necessarily entails a metaphysical gap.
Others, such as Joseph Levine, have wished to either remain silent on the matter or argue that no such metaphysical conclusion should be drawn. He agrees that conceivability (as used in the Zombie and inverted spectrum arguments) is flawed as a means of establishing metaphysical realities; but he points out that even if we come to the metaphysical conclusion that qualia are physical, they still present an explanatory problem.
While I think this materialist response is right in the end, it does not suffice to put the mind-body problem to rest. Even if conceivability considerations do not establish that the mind is in fact distinct from the body, or that mental properties are metaphysically irreducible to physical properties, still they do demonstrate that we lack an explanation of the mental in terms of the physical.
However, such an epistemological or explanatory problem might indicate an underlying metaphysical issue—the non-physicality of qualia, even if not proven by conceivability arguments is far from ruled out.
In the end, we are right back where we started. The explanatory gap argument doesn't demonstrate a gap in nature, but a gap in our understanding of nature. Of course a plausible explanation for there being a gap in our understanding of nature is that there is a genuine gap in nature. But so long as we have countervailing reasons for doubting the latter, we have to look elsewhere for an explanation of the former.
See also 
- Animal consciousness
- Functionalism (philosophy of mind)
- Hard problem of consciousness
- Mind-body problem
- Philosophical zombie
- Philosophy of mind
- Problem of other minds
- Reverse engineering
- Turing test
- Levine, J. 1983. “Materialism and qualia: the explanatory gap”. Pacific Philosophical Quarterly, 64: 354-361.
- David Chalmers, Facing Up to the Problem of Consciousness, JCS, 2 (3), 1995, pp. 200-19.
- Dennett, D. C. 1991. Consciousness Explained. Boston: Little, Brown and Company.
- McGinn, C. 1989. “Can we solve the mind-body problem?” Mind, 98: 349-66
- Chalmers, D. 1996. The Conscious Mind. Oxford: Oxford University Press.
- Levine, J. Conceivability, Identity, and the Explanatory Gap
- Joseph Levine's homepage with On-Line Articles on the problem
- What is it like to be a bat? Thomas Nagel, The Philosophical Review LXXXIII, 4 (October 1974): 435-50.