Explanatory Gap

The MIND-BODY PROBLEM -- the problem of understanding the relation between physical and mental phenomena -- has both a metaphysical side and an epistemological side. On the metaphysical side, there are arguments that purport to show that mental states could not be (or be realized in) physical states, and therefore some version of dualism must be true. On the epistemological side, there are arguments to the effect that even if in fact mental states are (or are realized in) physical states, there is still a deep problem about how we can explain the distinctive features of mental states in terms of their physical properties. In other words, there seems to be an "explanatory gap" between the physical and the mental (see Levine 1983 and 1993).

The distinctive features that seem to give rise to the explanatory gap are the qualitative characters of conscious experiences (or QUALIA), such as the smell of a rose or the way the blue sky looks on a clear day. With conscious creatures it seems sensible to ask, with regard to their conscious mental states, WHAT-IT'S-LIKE to have them. The answers to these questions refer to properties that seem quite unlike the sorts of properties described by neurophysiologists or computational psychologists. It is very hard to see how the qualitative character of seeing blue can be explained by reference to neural firings, or even to the information flow that encodes the spectral reflectance properties of distal objects. It always seems reasonable to ask, but why should a surface with this specific spectral reflectance look like that (as one internally points at one's experience of blue)? For that matter, it seems reasonable to ask why there should be anything it is like at all to see blue, inasmuch as detecting and encoding information about the external world does not automatically entail having a genuine experience. After all, thermometers and desktop computers detect and encode information, but few people are tempted to ascribe experience to them.

Traditionally, dualists have employed conceivability arguments to demonstrate a metaphysical distinction between the mental and the physical. Whether or not these arguments work to establish the metaphysical thesis of dualism (for arguments pro and con see Chalmers 1996; Jackson 1993; Block and Stalnaker forthcoming), they can be employed to support the existence of the explanatory gap. We start with the assumption that adequate explanations reveal a necessary relation between the factors cited in the explanation (the explanans) and the phenomenon to be explained (the explanandum). For example, suppose we want to know why water boils at 212° F. at sea level. Given the molecular analysis of water, boiling, and temperature, together with the relevant physical and chemical laws, it becomes apparent that under these conditions water just has to boil. The point is, we can see why we should not expect anything else.

Contrast this example with what it is like to see blue. After an exhaustive specification of both the neurological and the computational details, we really do not know why blue should look the way it does, as opposed, say, to the way red or yellow looks. That is, we can still conceive of a situation in which the very same neurological and computational facts obtain, but we have an experience that is like what seeing red or yellow is like. Because we can imagine a device that processed the same information as our visual systems but was not conscious at all, it is also clear that we do not really know why our systems give rise to conscious experience of any sort, much less of this specific sort. Again, the contrast with the boiling point of water is instructive. Once we fill in the appropriate microphysical details, it does not seem conceivable that water should not boil at 212° F. at sea level.

One final example is quite helpful to make the point. Frank Jackson (1982) imagines a neuroscientist, Mary, who knows all there is to know about the physical mechanisms underlying color vision. However, she has been confined all her life to a black and white environment. One day she is allowed to emerge into the world of color and sees a ripe tomato for the first time. Does she learn anything new? Jackson claims that obviously she does; she learns what red looks like. But if all the information she had before really explained what red looked like, it seems as if she should have been able to predict what it would look like. Thus her revelation on emerging from the colorless world supports the existence of the explanatory gap.

There are various responses to the explanatory gap. One view (see McGinn 1991) is that it reflects a limitation on our cognitive capacities. We just do not have, and are constitutionally incapable of forming, the requisite concepts to bridge the gap. Others argue that the gap is real but that it is to be expected given certain peculiarities associated with our first-person access to experience (see Lycan 1996; Rey 1996; Tye 1995). Just as one cannot derive statements involving indexicals from those that do not -- e.g., "I am here now" from "Joe Levine is at home on Saturday, June 27, 1997" -- so one cannot derive statements containing terms like "the way blue looks" from those that contain only neurological or computational terms. Still others (see Churchland 1985) argue that advocates of the explanatory gap just do not appreciate how much one could explain given a sufficient amount of neurological detail. Finally, some see in the explanatory gap evidence that the very notion of qualitative character at issue is confused and probably inapplicable to any real phenomenon (see Dennett 1991 and Rey 1996). On this view qualia literally do not exist. Although we have experiences, they do not actually possess the features we naively take to be definitive of them.

There are complex and subtle issues involved with each of these responses. For instance, the precise role of identity statements in explanations, and the degree to which identities themselves are susceptible of explanation, must be explored more fully. (For a lengthy discussion of the conceivability argument that deals with these issues, see Levine forthcoming.) Suffice it to say that no consensus yet exists on the best way to respond to the explanatory gap.

See also

Additional links

-- Joseph Levine


Block, N. J., and R. Stalnaker. (Forthcoming). Conceptual Analysis and the Explanatory Gap.

Chalmers, D. (1996). The Conscious Mind. Oxford: Oxford University Press.

Churchland, P. (1985). Reduction, qualia, and the direct introspection of brain states. Journal of Philosophy 82:8-28.

Dennett, D. C. (1991). Consciousness Explained. Boston: Little, Brown.

Jackson, F. (1982). Epiphenomenal qualia. Philosophical Quarterly 32:127-136.

Jackson, F. (1993). Armchair metaphysics. In J. O'Leary-Hawthorne and M. Michael, Eds., Philosophy in Mind. Dordrecht: Kluwer.

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 and G. Humphreys, Eds., Consciousness: Psychological and Philosophical Essays. Oxford: Blackwell, pp. 121-136.

Levine, J. (Forthcoming). Conceivability and the metaphysics of mind.

Lycan, W. G. (1996). Consciousness and Experience. Cambridge, MA: Bradford Books/MIT Press.

McGinn, C. (1991). The Problem of Consciousness. Oxford: Blackwell.

Rey, G. (1996). Contemporary Philosophy of Mind: A Contentiously Classical Approach. Oxford: Blackwell.

Tye, M. (1995). Ten Problems of Consciousness: A Representational Theory of the Phenomenal Mind. Cambridge, MA: Bradford Books/MIT Press.

Further Readings

Clark, A. (1993). Sensory Qualities. Oxford: Oxford University Press.

Dretske, F. (1995). Naturalizing the Mind. Cambridge, MA: Bradford Books/MIT Press.

Flanagan, O. (1992). Consciousness Reconsidered. Cambridge, MA: Bradford Books/MIT Press.

Hardcastle, V. G. (1995). Locating Consciousness. Amsterdam/Philadelphia: John Benjamins.

Hardin, C. L. (1987). Qualia and materialism: closing the  explanatory gap. Philosophy and Phenomenological Research 47(2).

Levin, J. (1983). Functionalism and the argument from conceivability. Canadian Journal of Philosophy, sup. vol. 11.

Loar, B. (1990). Phenomenal states. In J. Tomberlin, Ed., Action Theory and Philosophy of Mind; Philosophical Perspectives 4. Atascadero, CA: Ridgeview Publishing Co., pp. 81-108.

Metzinger, T., Ed. (1995). Conscious Experience. Paderborn: Ferdinand Schöningh/ Imprint Academic.

Yablo, S. (1993). Is conceivability a guide to possibility? Philosophy and Phenomenological Research 53(1):1-42.