The problem of experience
Philosopher David Chalmers works as a University Professor of Philosophy and Neural Science at New York University. He is known for describing what he termed the "hard problem of consciousness." He first presented this formulation at the inaugural "Toward a Science of Consciousness" conference in Tucson, Arizona, in April 1994, and expanded on it in his 1996 book, The Conscious Mind.
The hard problem asks: Why does the brain's processing of information give rise to subjective, qualitative experience? In philosophy, these individual instances of subjective experience, the perceived redness of red, the taste of wine, or the pain of a headache, are called "qualia." Chalmers contrasts the hard problem with the "easy problems" of consciousness. The easy problems concern the brain's functions: how it processes stimuli, integrates information, controls behavior, and generates verbal reports. These problems are considered "easy" not because they are simple to solve, but because they are ultimately explainable by discovering the right neural or computational mechanisms. The hard problem persists even after all these functional explanations are complete, leaving the question of why these functions are accompanied by a feeling, or "what it is like" to be that organism.
Philosophical zombies and the explanatory gap
To demonstrate the distinction between the physical brain and subjective experience, Chalmers employs a thought experiment involving a "philosophical zombie" or "p-zombie." A p-zombie is a hypothetical being that is a molecule-for-molecule duplicate of a conscious person. It behaves identically to a human, reacts to stimuli, reports internal states, and appears to have a complex emotional life, but it has no actual subjective experience. If a p-zombie were poked with a sharp object, it would flinch and say "ouch," but it would not feel any pain.
The conceivability of such a zombie, Chalmers argues, shows that consciousness is an additional fact about the world, beyond all the physical facts. If physicalism—the theory that everything in the universe is physical—were true, then a physical duplicate of a person would necessarily be a conscious duplicate. The logical possibility of a p-zombie suggests that physical properties alone do not determine conscious experience, creating what philosopher Joseph Levine termed an "explanatory gap" between physical processes and mental feelings.
Not all philosophers agree. Daniel Dennett, for instance, argues that the concept of a philosophical zombie is incoherent. He suggests that any system complex enough to perfectly mimic human behavior would, by necessity, have the same kind of consciousness we do. From this perspective, the "hard problem" is a result of a conceptual confusion, and what we call consciousness is the sum of the brain's many complex functions, a "bag of tricks."