Thomas Nagel, answered from the texts and cited to the page.
There is something it is like to be a bat. There is something it is like to be each conscious creature, and the subjective character of conscious experience is what physicalism, in attempting to reduce or eliminate the mental, must miss.12 A physicalist account of bat neurology gives us, at most, knowledge of the objective character of the bat's brain states; what such an account cannot give us is what the bat's experience is like for the bat.
The fact that we cannot know what bat-experience is like (because the bat's perceptual modalities are too different from ours) does not show that there is nothing there to be known; it shows that our objective methods cannot reach the subjective character of an experience whose perspective we do not share. The argument generalizes: phenomenal consciousness has a subjective character that is essentially perspectival, and the reduction-elimination-annexation strategies of physicalism cannot capture it.
The bat thesis is not the claim that consciousness is non-physical in some Cartesian-dualist sense; it is the claim that physicalist methods, as currently understood, do not have the conceptual resources to handle the subjective character of experience. Future understanding will require a richer framework — one of the projects of Mind and Cosmos (2012) is to gesture at what such a framework might look like.345
There may be further implications about the form of the experience; there may even (though I doubt it] e implications about the behavior of the organism. But 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/or the organism. We may call this the subjective character of experience.Mortal Questions, pp. 180–181
The most radical thing about the present conjecture is the idea that there is something more fundamental than the physical—something > The Psychophysical Nexus 27. Nagel, "What Is It Like to Be a Bat?" 28. Jeffrey A. Gray, "The Contents of Consciousness: A Neuropsychological Conjecture," Behavioral and Brain Sciences, 18 (1995), 660. that explains both the physical and the mental.Concealment and Exposure, pp. 233–234
But that is not the question. I want to know what it is like for a bat to be a bat. Yet if I try to imagine this, I am restricted to the resources of my own mind, and those resources are inadequate to the task.Mortal Questions, pp. 183–184
Whatever may be the status of facts about what it is like to be a human being, or a bat, or a Martian, these appear to be facts that embody a particular point of view. I am not adverting here to the alleged privacy of experience to its possessor. The point of view in question is not one accessible only to a single individual.Mortal Questions, p. 186
Lawrence Foster and J. W. Swanson (Amherst: University of Massachusetts Press, 1970); though I do not understand the argument against psychophysical laws. 13 Similar remarks apply to my paper 'Physicalism, Philosophical Review, LXXIV (1965), 339-56, reprinted with postscript in Modern Materialism, cd. John O'Connor (New York: Harcourt Brace Jovanovich, 1969).Mortal Questions, pp. 192–193
"What Is It Like to Be a Bat?" (1974) is one of the most-cited essays in twentieth-century philosophy of mind and inaugurates the modern anti-physicalist tradition. Frank Jackson's 'Epiphenomenal Qualia' (1982) and the Mary's-room argument provide a parallel anti-physicalist case. David Chalmers's The Conscious Mind (1996) and 'Facing Up to the Problem of Consciousness' (1995) extend the bat thesis as the "hard problem of consciousness." The eliminativist response (Dennett's Consciousness Explained, Churchland's eliminative materialism, the Quining-qualia tradition) treats the subjective character as an illusion to be eliminated. The debate is not settled; the field divides between those who treat phenomenal consciousness as a real explanandum that physicalism has not explained and those who treat it as a residual confusion.