Zombies and Other Minds are both well-known philosophical topics, though Zombies are 'hotter': the heyday of Other Minds debates was back in the '50s and '60s. How are they related? Not by identity, though some do confuse the two topics. They are distinct in that Other Minds belongs to epistemology whereas Zombies belongs to ontology. Let me see if I can work this out in detail.
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 as in the picture above.) 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 irreducible intentionality, 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 zombie? There is nothing it is like to be a zombie.) I suspect that Daniel Dennett is a zombie. But I have and can have no evidence for this suspicion. His denial of qualia is not evidence. It might just be evidence of his being a sophist. More to the point, his linguistic behavior and facial expressions could be just the same as those of a non-zombie qualia-denier.
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 -- physicalists after all are benighted but not stupid -- 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 bloody objectivists: they think that anythng 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.
The truth of physicalism is not my main concern this Halloween. My main concern is merely to explain the role of the zombie Gedankenexperiment. The point is that zombies figure in discussions of the ontological thesis of physicalism. If zombies are possible, then physicalism is false.
4. Zombies and Other Minds
So what is the difference between the Zombie question and the Other Minds question? I'll have to think about this in greater depth, but here are some 'shoot-from-the hip' remarks of the sort one can get away with on a weblog.
Suppose Dennett is a zombie as I suspect. The poor creature has no inner life, no interiority. Since he has no mind in the sense of 'mind' in which I know that I have a mind, he cannot be an other mind to me or to any other minded individual. If an organism has no mind, then no question can arise as to how one knows or rationally believes that it has a mind, nor any question as to how one knows nor rationally believes that it is in one state of mind rather than another. It is these epistemological questions that arise within the context of discussions of Other Minds.
As I see it, if physicalism is true, then we are all zombies. We are all stumbling around 'in the dark.' But if so, then the epistemological problems associated with the Other Minds debate should have fairly easy solutions. If verbal and non-verbal behavior is constitutive of mentality, then I should be able to know that you are skeptical or angry or bored or fearful just by looking at your face, observing your 'body language,' and listening to your words. External criteria would suffice. But if we are not zombies, and physicalism is false, then the epistemological questions regain their urgency.
Whatever the exact details, the Zombie and Other Minds questions are not to be conflated.
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