In 1994, at the inaugural Toward a Science of Consciousness conference in Tucson, Arizona, a young Australian philosopher named David Chalmers delivered a paper that reoriented the entire field of philosophy of mind. The paper, subsequently expanded into his 1996 book 'The Conscious Mind: In Search of a Fundamental Theory,' introduced the distinction between what Chalmers called the 'easy problems' of consciousness and the 'hard problem.' The easy problems — despite the name — are genuinely difficult scientific problems. They include: how the brain integrates information from multiple sensory sources into a unified percept; how the brain directs attention; how we report our mental states; why some neural processes are accompanied by awareness and others are not; how the brain controls behavior. These problems are 'easy' not because they have been solved but because they are of the right type to be solved: given sufficient understanding of the brain's mechanisms, we can in principle explain all of these functions. The hard problem is different in kind.
The hard problem asks: why is there subjective experience at all? Even if we had a complete functional account of the brain — every neuron mapped, every synaptic connection documented, every information-processing algorithm identified — we would still face the further question: why does all of this processing give rise to inner experience? Why isn't it all conducted 'in the dark,' without any accompanying feel? Why does the activation of certain visual cortex neurons produce the experience of seeing red rather than merely the information processing appropriate to the detection of a specific wavelength of light? The technical name for the subjective, first-person character of experience is 'qualia' — the redness of red, the painfulness of pain, the taste of coffee. Qualia are the raw feels of experience, and they are precisely what no purely functional account of the brain seems able to explain. You can describe the function of pain reception in complete neurological detail without mentioning the fact that it hurts.
Joseph Levine's 'explanatory gap' (1983) articulated the problem before Chalmers gave it its canonical name: even if we accept that brain state X causes or is identical to experience Y, we cannot see WHY that particular brain state produces THAT particular experience rather than some other, or no experience at all. The gap between the neurological description and the experiential description seems irreducible — not merely a gap in our current knowledge but a gap in the conceptual framework that scientific explanation employs. Chalmers' 'zombie argument' makes this vivid: imagine a physical duplicate of yourself — atom for atom identical, behaving in every way exactly as you do — but with no inner experience at all. This 'philosophical zombie' processes information, reports on its mental states, screams when injured — but there is nothing it is like to be a zombie. No inner feel accompanies any of its processing. Chalmers argues that such a zombie is conceivable — we can imagine it without contradiction — and that therefore the conceivability argument establishes that consciousness cannot be purely physical, because a physical duplicate without consciousness is conceivable.
The zombie argument is among the most debated in philosophy of mind. Eliminative materialists like Daniel Dennett argue that the argument fails because philosophical zombies are not genuinely conceivable — that Chalmers is confusing our current inability to explain consciousness with a genuine metaphysical gap. Type B materialists acknowledge the explanatory gap but argue it is purely epistemic (a gap in our knowledge) rather than ontological (a gap in reality). Property dualists like Chalmers himself argue that the gap reveals that consciousness is a fundamental feature of reality, not reducible to physical processes. Panpsychists argue that consciousness is a fundamental property of all matter, not an emergent property of complex neural systems. Each of these positions has sophisticated defenders and raises its own deep problems — which is precisely what makes the hard problem hard. It is not merely a puzzle within an existing framework. It is a crack in every existing framework.