Chalmers is best known for formulating what he calls the "
hard problem of consciousness," in both his 1995 paper "Facing Up to the Problem of Consciousness" and his 1996 book
The Conscious Mind. He makes a distinction between "easy" problems of consciousness, such as explaining object discrimination or verbal reports, and the single hard problem, which could be stated "why does the
feeling which accompanies awareness of sensory information exist at all?" The essential difference between the (
cognitive) easy problems and the (
phenomenal) hard problem is that the former are at least theoretically answerable via the dominant strategy in the philosophy of mind:
physicalism. Chalmers argues for an "explanatory gap" from the objective to the subjective, and criticizes physicalist explanations of mental experience, making him a
dualist. Chalmers characterizes his view as "
naturalistic dualism": naturalistic because he believes mental states
supervene "naturally" on physical systems (such as brains); dualist because he believes mental states are ontologically distinct from and not reducible to physical systems. He has also characterized his view by more traditional formulations such as
property dualism.
In support of this,
Chalmers is famous for his commitment to the logical (though, not natural) possibility of philosophical zombies.[13] These zombies, are complete physical duplicates of human beings, lacking only qualitative experience. Chalmers argues that since such zombies are conceivable to us, they must therefore be logically possible. Since they are logically possible, then
qualia and sentience are not fully explained by physical properties alone; the facts about them are
further facts. Instead, Chalmers argues that consciousness is a fundamental property
ontologically autonomous of any known (or even possible) physical properties,
[14] and that there may be lawlike rules which he terms "psychophysical laws" that determine which physical systems are associated with which types of qualia. He further speculates that all
information-bearing systems may be conscious, leading him to entertain the possibility of conscious thermostats and a qualified
panpsychism he calls
panprotopsychism. Chalmers maintains a formal agnosticism on the issue, even conceding that the viability of panpsychism places him at odds with the majority of his contemporaries. According to Chalmers, his arguments are similar to a line of thought that goes back to
Leibniz's 1714
"mill" argument; the first substantial use of philosophical "zombie" terminology may be
Robert Kirk's 1974 "Zombies vs. Materialists".