The "explanatory gap" is the term introduced by philosopher Joseph Levine(1) for the alleged difficulty that a materialist (aka physicalist) theory of mind has in explaining how physical properties, materialist causal laws, and physical events can give rise to the phenomenal aspects of the way things feel when they are consciously experienced. According to many philosophers, there is this explanatory gap between the physical truths of the world, and the phenomenal truths of subjective experience. According to the proponents of the explanatory gap, whatever materialist account we might imagine of subjective conscious experience will leave it completely puzzling why there should be such a connection between the objective physical story and the subjective conscious experience(2). According to David Chalmers, finding an acceptable materialist explanation for the subjectiveness of experience is "the hard problem" of Philosophy of Mind(3).
There is a weak understanding of the "gap", and a strong understanding. The weak version of the explanatory gap is an epistemological concept -- it regards the explanatory gap as resulting from a difference between two kinds of understanding, objective and subjective. A search for an objective understanding characterizes the physical sciences - physics, chemistry, biology, geology, and so on. These look at things objectively, describing what their functions are, what they are composed of, how they operate. Physical science aims to uncover causal laws and behavioral regularities involving things and their parts. The aim is to achieve an understanding of phenomena "from the outside." Subjective phenomena do not play a role in the physical sciences.
But subjective phenomenology does play a major role in the sciences of the mind. These sciences are concerned, certainly, with objective understanding. But they also make use of a different sort of subjective understanding - "from the inside." There are aspects of reasons, purposes, feelings, thoughts, and experiences that can only be understood from within, via sympathy or empathy or other translation into one's own experience(4). Broadly understood subjective phenomena (intentions, desires, and so forth) play a pivotal role in sociology, economics, political theory, anthropology, history, and psychology. As well, of course, in our ordinary "folk" psychological attributions of motivations and beliefs to other minds.
The weak (epistemological) version of the explanatory gap labels the obvious fact that we do not yet have an acceptable materialist (objective, from the outside) theory of how physical properties, materialist causal laws, and physical events can give rise to a first-person subjective experience of conscious phenomena. Think of a modern computer, by comparison. No one has a problem of agreeing that a computer's behavior can be fully explained by their circuitry, and materialist causal laws. Any attribution to the computer of reasons, purposes, feelings, thoughts, or experiences is readily and universally understood as anthropomorphic. There is no explanatory gap here. Not so when we do the same kind of attribution to other people, or even to ourselves.
Understood epistemologically, the explanatory gap is readily accepted by materialists, and merely recognizes a gap in our understanding of how the material causes the subjective, or how the subjective supervenes on the physical. Even if we come to (or start from) the metaphysical conclusion that qualia and other subjective phenomena are physical, there still remains an epistemological explanatory gap. In other words, an epistemological "explanatory gap" does not cast any doubt on the truth of materialism.
The strong version of the explanatory gap, on the other hand, is a metaphysical concept. Proponents of the strong understanding of the gap claim that the mind is substantially and qualitatively different from the brain and that the existence of something metaphysically extra-physical is required. It is thought by many mind-body dualists (e.g. René Descartes, David Chalmers, Colin McGinn) that subjective conscious experience constitutes a separate effect that is outside the materialist world. Thus there is a gap between the explanations that can be offered by the materialists, and the explanations that must be offered to explain subjective phenomena.
The current inability of materialists to supply a suitably intelligible explanation of subjective conscious experience in the same manner as the explanation we could offer for computer phenomena is considered by many anti-materialists to be evidence of the existence of a metaphysical explanatory gap. For example, Nagel(2) and Jackson(5) take the gap as evidence that objective physical explanations cannot account for the intrinsic quality of experience. Although Jackson(6) changes his mind and later joins the ranks of materialist philosophers who deny that there is such a metaphysical gap. Searle(7) argues that the gap between any imaginable functionalist account and the intrinsic intentionality of thoughts is evidence against a functionalist explanation of that intrinsic intentionality. Chalmers(8) takes the existence of the gap to be evidence against materialism in favor of some sort of dualism. And McGinn(9) sits on the fence, as it were. He suggests that the gap is due to inherent limitations on the powers of human understanding - we cannot comprehend whatever might be necessary to bridge the gap. Clearly not a materialist position, yet not obviously a dualist one either.
There is no general consensus regarding whether the metaphysical gap exists, or what metaphysical conclusions the existence of an epistemological gap provides. Those wishing to use its existence to support dualism (or at least to deny materialism) take 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(9), argue that no such metaphysical conclusion should be drawn.
Some anti-materialists have appealed to conceivability arguments for support. The most famous are those of Jackson's Mary(5), Chalmers' Zombies(3, 10), and the Inverted Spectrum(11) argument. Such "knowledge arguments" as they are called do not obviously avoid begging the anti-materialist question. But they rely upon claims and intuitions that are controversial and not completely independent of one's basic view about physicalism. Many materialist philosophers argue that the conceivability argument (as used in the Mary, Zombie, and inverted spectrum thought experiments) is fatally flawed as a means of establishing that the epistemological gap is also a metaphysical gap. The materialist response to these conceivability arguments focuses on the fact that how one thinks of "conceivability" will depend on one's metaphysical pre-commitments. If one is pre-committed to materialism, then the "conceivability" involves a logical contradiction, and conceivability cannot imply possibility (a logical contradiction is not possible). If one is pre-committed against materialism, then the "conceivability" does not involve a logical contradiction and can imply possibility (maybe, depending on the details of the notions of conceivability and possibility involved(12)). And one cannot sit on the fence on this issue -- otherwise the concept of "conceivability" employed lacks foundation and can prove nothing. So either way, these conceivability arguments beg the question. However, debate on the topic remains active and ongoing.
If one could establish on a priori grounds that there is no way in which subjective phenomena could be intelligibly explained in materialist terms, it would be acceptable to conclude that the epistemological gap is also a metaphysical gap. However, the very strength of such a claim makes it difficult to assume without begging the metaphysical result in question. Thus those who wish to use the epistemological gap to refute metaphysical physicalism must find independent grounds to support it.
Whether this metaphysical gap exists thus depends on the prior commitments of the philosopher in question. Thus, a metaphysical "explanatory gap" does not cast doubt on the truth of materialism. Rather, whether one acknowledges the very existence of the gap depends on a prior commitment to the falsity of materialism.
In the end, we are right back where we started. The epistemological explanatory gap argument doesn't demonstrate a metaphysical gap in 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.(9)
(1) Levine, Joseph; "Materialism and Qualia: the Explanatory Gap" in Pacific Philosophical Quarterly, Vol 64 (1983). Pp 354-361.
(2) Nagel, Thomas; "What Is It Like to be a Bat?" in Philosophical Review, Vol 83 (1974). Pp 435-450.
(3) Chalmers, David; "Facing Up to the Problem of Consciousness" in Journal of Consciousness Studies, Vol 2, No 3 (1995). Pp 200-19.
(4) Harman, Gilbert; "Explaining an Explanatory Gap," in American Philosophical Association Newsletter on Philosophy and Computers, Vol 6, No 2 (2007). Pp 2-3
(5) Jackson, Frank; "Epiphenomenal Qualia" in Philosophical Quarterly, Vol 32 (1982). Pp 127-132.
"What Mary Didn't Know" in Journal of Philosophy, Vol 83 (1986). Pp 291-295.
(6) Jackson, Frank; "Postscript" in Contemporary Materialism, P. Moser & J. Trout (eds). Routledge, London, England, 1995. Pp 184-189.
"Postscript on Qualia" in his Mind, Method, and Conditionals. Routledge, London, England, 1998. Pp 76-79.
"Mind and Illusion" in There's Something About Mary: Essays on Phenomenal Consciousness and Frank Jackson's Knowledge Argument, Ludlow, Nagasawa, Stoljar (eds). Pp 421-442.
(7) Searle, John; Minds, Brains, and Science. Harvard University Press, Cambridge, Mass, 1986. ISBN 978-0-674-57633-9.
(8) Chalmers, David; The Conscious Mind: In Search of a Fundamental Theory. Oxford University Press, Oxford, England, 1997. ISBN 978-0-195-11789-9.
(9) McGinn, Colin; "Can we solve the mind-body problem?" in Mind, Vol 98 (1989). Pp 349-66
Problems in Philosophy: The Limits of Inquiry. Wiley-Blackwell, Oxford, England, 1993. ISBN 978-1-5578-6475-8.
"How Not To Solve the Mind-Body Problem" in Physicalism and Its Discontents, Gillett, Loewer (eds). Pp 284-306.
(9) Levine, Joseph; "Conceivability, Identity, and the Explanatory Gap" in Towards a Science of Consciousness III: The Third Tucson Discussions and Debates, Hameroff, Kaszniak, Chalmers (Eds). Pp 3-12.
(10) Kirk, Robert, "Zombies" in The Stanford Encyclopedia of Philosophy (Summer 2012 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/sum2012/entries/zombies/>.
(11) Byrne, Alex; "Inverted Qualia" in The Stanford Encyclopedia of Philosophy (Spring 2010 Edition), Edward N. Zalta (ed.), URL=<http://plato.stanford.edu/archives/spr2010/entries/qualia-inverted/>.
(12) Bailey, Andrew; "The Unsoundness of Arguments From Conceivability", 2007, URL=<http://www.uoguelph.ca/~abailey/Resources/The%20Unsoundness%20of%20Arguments%20from%20Conceivability%20(Latest%20Draft).pdf>
Block, Ned & Stalnaker, Robert; "Conceptual Analysis, Dualism and the Explanatory Gap," in The Philosophical Review, Vol 10, No 3 (1999), Pp 315-360;
Gillett, Carl & Loewer, Barry (eds.); Physicalism and Its Discontents. Cambridge University Press. Cambridge, England, 2001. ISBN 978-0-521-80175-1.
Hameroff, Stuart R. & Kaszniak, Alfred W. & Chalmers, David (Eds): Towards a Science of Consciousness III: The Third Tucson Discussions and Debates, A Bradford Book, The MIT Press, Cambridge, Mass. 1999. ISBN 978-0-2625-8181-3.
Hardin, C. L.; "Qualia and Materialism: Closing the Explanatory Gap" in Philosophy and Phenomenological Research, Vol 48 No 2 (1987) Pp 281-98.
Harman, Gilbert; "Can Science Understand the Mind?" in Conceptions of the Human Mind: Essays in Honor of George A. Miller, G. Harman (ed.). Lawrence Erlbaum, Hillsdale, N.J., 1993: Pp 111-121.
Harman, Gilbert; "Immanent and Transcendent Approaches to Meaning and Mind" in his Reasoning, Meaning, and Mind. Oxford University Press ,Oxford, England, 1999. Pp 262-275.
Polycn, Karol; "Phenomenal Consciousness and Explanatory Gap" in Diametros, Vol 6 (2005), Pp 62-52;
Tye, Michael: Consciousness Revisited: Materialism without Phenomenal Concepts, MIT Press, Cambridge, Mass. 2009. Pg 237. ISBN 0-262-01273-1. Kindle Edition.
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