He is a philosopher of mind famous for his (1983) insight into the difficulty that a physicalist theory has in explaining how material properties of a brain can account for the way things consciously feel when they are experienced. He called it the "explanatory gap."

Philosophers of mind ask how any objective physical process can ever account for our personal, internal, and subjective experiences, which they call "qualia."

Levine's "explanatory gap" is at the heart of what David Chalmers later (1996) called "the hard problem of consciousness."

Most simply put, the hard problem is a subset of the more general mind-body problem, which, since René Descartes, asks how an immaterial mind can have any causal connection with the material body, and vice versa.

This is a problem with dualisms that separate the world into matter (materialism) and ideas (idealism). Pure idealists deny the existence of matter. Materialists reduce mental states to physical states. Eliminative materialists deny the existence of conscious minds. Behavioral psychologists identify the mind with the brain.

The first philosophers to argue for an identity of mind (or consciousness) and the brain include Ullin T. Place (1956), Herbert Feigl (1958), and J.J.C.Smart (1959).

Place explicitly describes "consciousness as a brain process," specifically as "patterns" of brain activity. He does not trivialize this identity as a succession of individual "mental events and physical events" in some kind of causal chain. He compares this identity to the idea that "lightning is a motion of electrical charges."

("Is Consciousness a Brain Process?", in British Journal of Psychology, 47, pp.44-50 (1956))

Herbert Feigl's work was independent of Place's, but he said that the fundamental idea had been held by many earlier materialist (monist) thinkers. He thought it was stated clearly by Rudolf Carnap in 1925. Feigl describes his own thesis:

The identity thesis which I wish to clarify and to defend asserts that the states of direct experience which conscious beings "live through" and those which we confidently ascribe to some of the higher animals, are identical with certain (presumably configurational) aspects of the neural processes in these organisms.

("The 'Mental' and the 'Physical'", in Concepts, Theories, and the Mind-Body Problem, University of Minnesota Press (1958), p.150)

J.J.C.Smart clarified and extended the identity theory of his colleague U.T.Place

When I say that a sensation is a brain process or that lightning is an electric discharge, I am using "is" in the sense of strict identity. (Just as in the — in this case necessary — proposition "7 is identical with the smallest prime number greater than 5.") When I say that a sensation is a brain process or that lightning is an electric dis- charge I do not mean just that the sensation is somehow spatially or temporally continuous with the brain process or that the lightning is just spatially or temporally continuous with the discharge.

("Sensations and Brain Processes", in Philosophical Review 68 pp. 141-156 (1959))

Levine criticizes these attempts to identify mind with or reduce it to brain processes.

On the one hand, it’s really hard to see how the reddish-orange character of seeing a sunset could be reducible to, or completely explicable in terms of, the neurological processes of the visual system. How does neural firing, no matter how complex, amount to a reddish-orange experience? On the other hand, analysing the qualitative character of experience in purely functional terms presents its own difficulties. Again, how does a formal property such as occupying a particular causal role or a point in a multidimensional space, amount to the seemingly quite concrete reddish-orange character of seeing a sunset? True, both traditional functionalism and psychoneural reductionism have their adherents,2 but their inadequacies are clearly felt by many philosophers.

(Knowing What It's Like, p.45)

Levine used Saul Kripke's sophisticated arguments in symbolic logic about the necessity of identity statements and Gottlob Frege's theory of reference and sense/meaning. He transformed Kripke's "metaphysical" argument into an epistemological argument.

In “Naming and Necessity” and “Identity and Necessity,” Kripke presents a version of the Cartesian argument against materialism. His argument involves two central claims: first, that all identity statements using rigid designators on both sides of the identity sign are, if true at all, true in all possible worlds where the terms refer; second, that psycho-physical identity statements are conceivably false, and therefore, by the first claim, actually false.

My purpose in this paper is to transform Kripke’s argument from a metaphysical one into an epistemological one. My general point is this. Kripke relies upon a particular intuition regarding conscious experience to support his second claim. I find this intuition important, not least because of its stubborn resistance to philosophical dissolution. But I don’t believe this intuition supports the metaphysical thesis Kripke defends — namely, that pyscho-physical identity statements must be false. Rather, I think it supports a closely related epistemological thesis — namely, that psycho-physical identity statements leave a significant explanatory gap, and, as a corollary, that we don’t have any way of determining exactly which psycho-physical identity statements are true. One cannot conclude from my version of the argument that materialism is false, which makes my version a weaker attack than Kripke’s. Nevertheless, it does, if correct, constitute a problem for materialism, and one that I think better captures the uneasiness many philosophers feel regarding that doctrine.

("Materialism and Qualia: The Explanatory Gap," Pacific Philosophical Quarterly, p.354)

A decade before Levine's "Explanatory Gap," Thomas Nagel's famous essay "What It's Like to be a Bat" homed in on the subjective "feelings" that the bat is experiencing as "qualia." Nagel opposed all attempts to reduce consciousness and mental actions to material (or "physical") explanations. He wrote:

Consciousness is what makes the mind-body problem really intractable. Perhaps that is why current discussions of the problem give it little attention or get it obviously wrong. The recent wave of reductionist euphoria has produced several analyses of mental phenomena and mental concepts designed to explain the possibility of some variety of materialism, psychophysical identification, or reduction...

Every reductionist has his favorite analogy from modern science. It is most unlikely that any of these unrelated examples of successful reduction will shed light on the relation of mind to brain... I shall try to explain why the usual examples do not help us to understand the relation between mind and body-why, indeed, we have at present no conception of what an explanation of the physical nature of a mental phenomenon would be. Without consciousness the mind-body problem would be much less interesting. With consciousness it seems hopeless...

Conscious experience is a widespread phenomenon. It occurs at many levels of animal life, though we cannot be sure of its presence in the simpler organisms...But no matter how the form may vary, the fact that an organism has conscious experience at all means, basically, that there is something it is like to be that organism...fundamentally an organism has conscious mental states if and only if there is something that it is like to be that organism - something it is like for the organism.

We may call this the subjective character of experience. It is not captured by any of the familiar, recently devised reductive analyses of the mental, for all of them are logically compatible with its absence...Any reductionist program has to be based on an analysis of what is to be reduced. If the analysis leaves something out, the problem will be falsely posed. It is useless to base the defense of materialism on any analysis of mental phenomena that fails to deal explicitly with their subjective character... Without some idea, therefore, of what the subjective character of experience is, we cannot know what is required of physicalist theory...

The reason is that every subjective phenomenon is essentially connected with a single point of view, and it seems inevitable that an objective, physical theory will abandon that point of view...

I have said that the essence of the belief that bats have experience is that there is something that it is like to be a bat...

(What It's Like to be a Bat, reprinted in Mortal Questions, Nagel, T., Cambridge, 1979, p.165-180)

In 1993, Levine replied to Nagel with an article "On Leaving Out What It's Like."

Among the reasons for doubting the adequacy of physicalist theories of the mind is the charge that such theories must “leave out” the qualitative, conscious side of mental life. One problem with evaluating this objection to physicalism is that it is not clear just what physicalist theories are being charged with. What is it for a theory to “leave out” a phenomenon? My project in this chapter is threefold: First, I want to clarify the anti-physicalist charge of “leaving out” qualia, distinguishing between a metaphysical and an epistemological reading of the objection. Second, I will argue that standard anti-physicalist conceivability arguments fail to show that physicalist theories “leave out” qualia in the metaphysical sense. But, third, I will also argue that these conceivability arguments do serve to establish that physicalist theories “leave out” qualia in the epistemological sense, because they reveal our inability to explain qualitative character in terms of the physical properties of sensory states. The existence of this “explanatory gap” constitutes a deep inadequacy in physicalist theories of the mind

("On Leaving Out What It's Like," in The Nature of Consciousness, Block et al., p.544)

Knowing What It's Like, in Privileged Access: Philosophical Accounts of Self-knowledge, ed. Gurtler, B., Routledge, 2003, pp.45-54.

On Leaving Out What It's Like, in The Nature of Consciousness, ed. Block, N., Flanagan, O., and Güzeldere, G, MIT Press, 1997, pp.544-555