1. What is a zombie?
You will have gathered that a zombie is a creature of philosophical fiction conjured up to render graphic a philosophical issue and to throw certain questions in the philosophy of mind into relief. A zombie is a living being that is physically and behaviorally exactly like a living human being except that it lacks (phenomenal) consciousness. Cut a zombie open, and you find exactly what you would find were you to cut a human being open. And in terms of linguistic and nonlinguistic behavior, there is no way to tell a 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 French philosopher would call interiority. There is no irreducible subjectivity, no qualitative feel to the 'visual' processing; there is nothing it is like for a 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.)
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, 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, at a first approximation, 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 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 zombies. 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 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:
1. If physicalism is true, then every nonphysical property-instantiation supervenes upon a physical property-instantiation.
2. If zombies are possible, then it is not the case that every nonphysical property-instantiation supervenes upon a physical property-instantiation.
3. Zombies are possible.
Therefore
4. Physicalism is not true.
This is a valid argument the soundness of which rides on premise (3). Here is where the fight will come. Without questioning the validity of the argument the physicalist will run the argument in reverse. He will deny the conclusion and then deny (3). In effect, he will argue from (1) & (2) & (~4) to (~3). 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.
Now I find that absurd: it is a denial of that subjectivity which is properly accessed only via the irreducible first-person singular point of view. Nevertheless, I will have a devil of a time budging my materialist-functionalist interlocutor. Materialists are objectivists: they think that anything that is not objectively accessible in the third-person way just isn't there at all, or it if is 'there,' is not to be taken seriously.
Can one support (3) in a manner so compelling as to convince the recalcitrant materialist? After all, (3) is not self-evident. If it were self-evident, then we would have a 'knock-down' argument against physicalism. But there are few if any 'knock-down' (absolutely compelling) arguments in philosophy.
Now zombies are certainly conceivable. But it is not clear whether conceivability entails metaphysical (broadly logical) possibility, which is in play in (3). So it is not clear whether the conceivability of zombies is a compelling reason to reject physicalism. The question of the relation between conceivability and possibility is a difficult one. There is some discussion of this in the conceivability category.
Now here is what Galen Strawson has to say:
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 experienceless PPD of a currently experiencing human being is conceivable, and could possibly (or ‘in some possible world’) exist.
This is just blatant question-begging on Strawson's part. We can't know that it is conceivable that zombies exist?! 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 that we have excellent reason to think it is true. 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.
Strawson tells us that 'physical' is a natural kind term. What a strange idea! '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 things very different in nature than 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.
I don't know what this means. Perhaps Vlastimil can explain it to me.
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