1. What is a zombie?
A zombie is a creature of philosophical fiction conjured up to render graphic a philosophical issue and to throw into relief certain questions in the philosophy of mind. A (human) zombie is a living being that is physically and behaviorally exactly like a living human being except that it lacks conscious experience. Cut a zombie open, and you will find exactly what you would find were you to cut open a (conscious) human being. They are made of the same stuff. And in terms of linguistic and nonlinguistic behavior, there is no way to tell a (conscious) human being from a zombie. So don't think of something sleepy, or drugged, or comatose, or Halloweenish. When a zombie sees a tree, what is going on in the zombie's brain is a 'visual' computational process, but the zombie lacks what a Continental philosopher would call interiority. There is no irreducible subjectivity, no qualitative feel to the 'visual' processing; there is nothing it is like for a male zombie to see a female zombie or to desire her. What's it like to be a horny zombie? There is nothing it is like to be a horny zombie. Indeed, there is nothing it is like to be a zombie, period. Exploiting the ambiguity of ‘like,’ if one zombie likes another, there is nothing that this liking is like. The liking has no phenomenal feel, no felt quality.
2. Where do zombies come from?
Zombies surface within the context of discussions of physicalism. Physicalism is an ontological doctrine, a doctrine about what ultimately exists, about what exists in the most fundamental sense of 'exists.' The physicalist is committed to the proposition that everything, or at least everything concrete, is either physical or determined by the physical. To be a bit more precise, physicalism is usefully viewed as the conjunction of an 'inventory thesis' which specifies physicalistically admissible individuals and a 'determination thesis' which specifies physicalistically admissible properties. What the inventory thesis says is that every concretum is either a physical item or composed of physical items. As for the determination thesis, what it says is that physical property-instantiations determine all other property-instantiations; equivalently, every nonphysical property-instantiation supervenes on physical property-instantiations. This implies that all mental facts supervene upon physical facts. So if a being is conscious, then this mental fact about it supervenes upon, is determined by, its physical properties. This implies that there cannot be two beings, indiscernible with respect to all physical properties, such that the one is conscious while the other is not. This in turn rules out the possibility of a conscious human being’s having a zombie twin. For if physicalism is true, once the physical properties are fixed, the mental properties are also automatically fixed.
3. What useful work do zombies do?
If zombies are metaphysically (broadly logically) possible, then physicalism is false. That's their job: to serve as counterexamples to physicalism. For if zombies are possible, then it is not the case that every nonphysical property-instantiation supervenes upon and is thus determined by a physical property-instantiation: a zombie has all the same physical properties as its indiscernible non-zombie twin, but is not conscious. The possibility of zombies implies that consciousness is non-supervenient, something in addition to a being's physical makeup. So one anti-physicalist argument goes like this:
a) If physicalism is true, then every nonphysical property-instantiation supervenes upon a physical property-instantiation.
b) If zombies are possible, then it is not the case that every nonphysical property-instantiation supervenes upon a physical property-instantiation.
c) Zombies are possible.
Therefore
d) Physicalism is not true.
This is an obviously valid argument in that the conclusion follows from the premises. Given that (a) and (b) are each true by definition, the soundness of the argument rides on premise (c). Here is where the fight will come. Without questioning the validity of the argument, one sort of physicalist will run the argument in reverse. He will deny the conclusion and then deny (c). In effect, he will argue from (a) & (b) & (~d) to (~c). He will deny the very possibility of zombies. He will insist that anything that behaves just like a conscious person and has the 'innards' of a conscious person just is a conscious person.
Is it possible to reject (c) while at the same time holding that consciousness is real, and not illusory as eliminativists such as Daniel Dennett maintain?
Enter Galen Strawson.
4. Strawson on Zombies.
It is, finally, a mistake to think that we can know that ‘zombies’ could exist—where zombies are understood to be creatures that have no experiential properties although they are perfect physical duplicates (PPDs) of currently experiencing human beings like you and me.
The argument that PPD-zombies could exist proceeds from two premisses—[1] it is conceivable that PPD-zombies exist, [2] if something is conceivable, then it is possible. It is plainly valid, and (unlike many) I have no insuperable problem with [2]. The problem is that we can't know [1] to be true, and have no reason to think it is. To be a materialist is, precisely, to hold that it is false, and while materialism cannot be known to be true, it cannot be refuted a priori—as it could be if [1] were established. ‘Physical’, recall, is a natural kind term, and since we know that there is much that we do not know about the nature of the physical, we cannot claim to know that an experience-less PPD of a currently experiencing human being is conceivable, and could possibly (or ‘in some possible world’) exist.
Here is Strawson’s argument:
1) It is conceivable that PPD-zombies exist.
2) If something is conceivable, then it is possible.
Therefore
3) PPD-zombies are possible.
Therefore
4) Physicalism is false.
Accepting (2), Strawson rejects (1). Given his acceptance of (2), his rejection of (1) amounts to the rejection of the possibility, not merely the conceivability, of PPD-zombies. He tells us that we cannot know (1) to be true, and that we have no reason to think that (1) is true. But we do have a reason, namely that PPD-zombies are conceivable, and therefore, by his own admission, possible! He then goes on to say that materialism cannot be refuted a priori. But surely it can if the reason given is good.
It is quite clear that Strawson is begging the question. That zombies are conceivable is a very weak claim, and of course we can know it to be true, just by conceiving a zombie, whence it follows — given Strawson’s acceptance of (2) — that we have good reason to think that zombies are possible. Strawson simply begs the question by assuming that materialism is true. He also begs the question by claiming that materialism cannot be refuted a priori. If you grant (2), as Strawson does, then what we have is an a priori refutation of materialism.
My point is that Strawson has not conclusively refuted the anti-materialist/physicalist position. What we have here is a classical philosophical stand-off. It may be that if we knew more about the nature of matter, then we would understand how matter could think, feel, be conscious. I do not rule out that epistemic possibility. Given our current understanding, however, no one has any idea how matter could be conscious. So I hold that it is reasonable to believe that materialism about the mind is false. Strawson is blustering, as we found him blustering on the God question.
Strawson tells us that 'physical' is a natural kind term. 'Water,' 'gold, 'tiger' are uncontroversial natural kind terms. They succeed in referring to what they were introduced to refer to despite our knowledge or ignorance of the nature of what they refer to. The ancient Greeks thought water was an element; Dalton held it to be HO; we take it to be H2O. Water turned out to be a lot different than we thought, without prejudice to the reference of 'water.' So if 'physical' is a natural kind term, then it too can refer to something very different in nature from what we might have supposed. And so Strawson thinks that 'physical' can refer to what is irreducibly mental or experiential in whole or in part. In fact, Strawson allows that the physical — that which physics studies — could be wholly mental.
Could be! It is epistemically possible given what we know and how little we know. But then again it could be that God and the “extraordinary suffering” that Strawson takes as definitely ruling out the existence of God co-exist. If we knew more about God and evil then perhaps we would see how the two are logically compatible. Strawson tells us that belief in God “. . . is profoundly immoral: it shows contempt for the reality of human suffering, or indeed any intense suffering.” How is that any better than saying that the belief that we are wholly material in nature is immoral: it shows contempt for the dignity of persons?
It is a curious fact about philosophers that they are loathe to admit the insolubility of their problems and the inevitability of stand-offs. Almost all of them think that they are right and their opponents wrong. I conjecture that this is due in large part to a privileging of their respective positions, a privileging often justified in the practical realm but wholly out of place in the precincts of pure theory.
Privileging their positions, they stamp their feet, wax dogmatic and bluster, seemingly unflustered by the fact that their brilliant interlocutors disagree with them. This exchange between Strawson and Dennett is a good example.