In its recent history, the philosophy of mind has come to resemble an entry into the genre of Hammer horror or pulpy science fiction. These days it is unusual to encounter a major philosophical work on the mind that is not populated with bats, homunculi, swamp-creatures, cruelly imprisoned genius scientists, aliens, cyborgs, other-worldly twins, self-aware computer programs, Frankenstein-monster-like 'Blockheads,' or zombies. The purpose of this paper is to review the role in the philosophy of mind of one of these fantastic thought-experiments — the zombie — and to reassess the implications of zombie arguments, which I will suggest have been widely misinterpreted. I shall argue that zombies, far from being the enemy of materialism, are its friend; and furthermore that zombies militate against the computational model of consciousness and in favour of more biologically-rooted conceptions, and hence that zombie-considerations support a more reductive kind of physicalism about consciousness than has been in vogue in recent years.

I Philosophical Zombies

Zombies — of the philosophical rather than the Haitian or Hollywood variety — are theoretically constructed creatures stipulated to be identical in certain respects with ordinary human beings, but lacking in other respects.¹ Perhaps the most familiar member of the zombie family is that [End Page 481] non-actual but (putatively) possible creature which is functionally identical with a 'normal human being' but entirely lacking in phenomenal states, in states of experiential consciousness. Such a zombie, as is characteristic of the breed, would be entirely indiscernible from the regular folks among whom it walks — as Owen Flanagan and Thomas Polger have put it (1995), it could fool even the sharpest 'mental detector' — but there would be 'no lights on' inside its head; all would be dark inside. A zombie will sometimes behave exactly as if it were in pain, or in love, or enjoying a movie, but could actually occupy none of those states: it could never, ex hypothesi, actually feel pain or experience enjoyment.

As is sometimes — but not always — noticed, the notion of a zombie is not a unitary one, and subtle variations in the construction of one's zombie thought-experiments can have important ramifications for their philosophical consequences.² There are at least three scales along which zombies can vary. First, there can be different ways in which zombies may be stipulated to be identical with normal persons: in particular, they may be quark-for-quark physically identically with people; they may be functionally or computationally identical with people; or they may merely be behaviourally identical or indiscernible. Second, zombies may be constructed such that they differ from normal people in various ways: for example, they might be postulated to lack any psychological states at all, or to be missing some subset of the psychological such as intentional or phenomenal states. Finally, there can be important differences in the modality of zombie claims: to assert that creatures indiscernible from humans but lacking mentality are possible is not yet to specify whether zombies are merely logically possible, metaphysically possible, nomically or naturally possible, or physically possible. Zombies might be logically possible but nomically impossible, for example, if their existence is incompatible with laws holding in the actual world (and in nomically similar worlds) but consistent with different laws of nature [End Page 482] that hold in other possible worlds. Similarly, zombies might be physically possible but nomically impossible if the laws of nature in the actual world outrun the physical laws — that is, if there are natural laws which are not also physical laws — and if zombies are consistent with the holding of all the physical laws but not with the holding of all the natural laws.

What, then, are zombies for — what is the point of constructing these fictional characters? Zombie thought-experiments, properly constructed, are ideal limit cases which can be used to reveal and assess the consequences of...