The philosophical zombie
On the axis: The enigma of consciousness · a double without experience
Imagine a double physically identical to you, acting and speaking as you do, but feeling nothing: no one home inside. If such a double is even conceivable, matter does not suffice to fix experience; but does what we can imagine prove anything about what is possible?
Justification
Chalmers separates the easy problems of consciousness, which concern mechanisms and which science will solve, from the hard problem: why are those mechanisms accompanied by any experience at all? No functional explanation answers it, for describing what a system does leaves untouched what it is like to be one. He concludes that are not reducible to physical processes but are tied to them by laws of nature, and calls his position a naturalistic dualism: consciousness is a fundamental feature of the world, like mass or charge, not a separate substance. He also explores, without adopting it, another route to the same intuition: lodging experience in the intrinsic nature of the physical, of which physics describes only the structure. That option, close to Russellian monism and to panpsychism, remains for him a serious avenue, not the thesis he defends.
"Facing Up to the Problem of Consciousness" (1995) · The Conscious Mind (1996)
Justification
Dennett rejects as the tradition conceives them: intrinsic, ineffable, private and infallible properties, given to introspection. What exists are brain states and the real patterns to which our reports attach; introspection is not a gaze upon an inner stage but a source of reports, often confabulated, which his method, heterophenomenology, treats as data to be explained rather than as established facts. Consciousness is therefore not denied: it is the appearance of an inner glow that itself becomes the effect to be explained.
Human consciousness is just about the last surviving mystery.Consciousness Explained (1991), ch. 1, opening sentence
"Quining Qualia" (1988) · Consciousness Explained (1991)