Property dualism is the view that there are some mental properties that are neither reducible to nor supervenient upon physical properties.

The zombies argument

A Philosophical ‘zombie’ is a human physically identical to us in all respects but with no consciousness – no mind. Physicalism claims that there is only the physical. In that case, a physical duplicate of a conscious being must be alike in all respects, including being conscious. However, Chalmers claims that a physical duplicate without consciousness – a zombie – is possible because no contradiction arises in conceiving of it. Since Physicalism seems committed to the view that zombies are impossible, Chalmers argues physicalism is false.

Chalmers uses modal logic which is the attempt to argue from what is possible to what is actual. This employs the terminology of ‘possible worlds’. In this world, the laws of physics which govern the mind and body (psycho-physical laws) make it impossible for a physical duplicate of me to lack consciousness. So, a physical duplicate of me in this world must also be conscious.

No matter how different the laws of physics are, there can’t exist possible worlds with impossible objects such as triangles with 4 sides. We cannot conceive of such objects. However, Chalmers claims we can conceive of a philosophical zombie and therefore a ‘zombie world’ is metaphysically possible.

P1. Philosophical zombies are conceivable (no contradiction arises in the conception of them).
P2. What is conceivable is metaphysically possible (exists in a possible world).
C1. Therefore, philosophical zombies are metaphysically possible (they exist in a possible world).
P3. If philosophical zombies are metaphysically possible, phenomenal properties of consciousness are not reducible to nor supervenient on physical properties.
C2. Therefore, property dualism is true.

The issue that a ‘philosophical zombie’ is not conceivable

A physicalist can respond that if physicalism is true, a zombie world is not conceivable. Anyone who thinks they are conceiving of a physical duplicate of a conscious person with no consciousness is confused. This confusion results from their ignorance of how certain physical systems and processes just are consciousness (e.g. type/token identity theory).

A full account of how certain physical systems are consciousness will include an explanation of how phenomenal properties of consciousness are not irreducible qualia but reduce to or supervene on physical properties. Anyone who has that full account will be unable to conceive of a duplicate of that physical system without consciousness. So, it is only if physicalism is false that zombies are conceivable.

Integration: this issue attacks premise 1, that zombies are conceivable, for begging the question. This is because P1 is only true if physicalism is false. The zombie argument fails as it presupposes that physicalism is false to reach the conclusion that physicalism is false.

However, we do not have a full physical account of consciousness. So, we cannot credibly claim to know that conceiving of a philosophical zombie is confused. We can only deny P1 and claim that zombies are not conceivable if we know physicalism to be true, which as a response to the zombie argument is a question-begging assumption itself.

All we currently have to go on is whether there seems to be anything self-contradictory in the conception of a zombie. Chalmers argues that there is not.

Nonetheless, Zombies may not be conceptually self-contradictory, but that only shows that they are logically possible. C1 requires that zombies are metaphysically possible, which their logical coherence is insufficient to establish.

So, we do not currently know whether zombies are conceivable in a way which justifies their being metaphysically possible (required for inference to C1).

We do not have to affirm the negation of P1 in order to conclude that the zombie argument fails. We can simply point out that since the zombie argument is attempting to reach a conclusion, it has the burden of proof and therefore fails because P1 is a question-begging assumption.

Integration: P1 is only true if physicalism is false. The truth of P1 therefore assuming the truth zombie argument’s conclusion (C2). The zombie argument therefore begs the question. This issue does not require that physicalism is true so it is not question-begging itself.

The issue that what is conceivable may not be metaphysically possible

Even if we can conceive of a zombie world, that does not mean it is possible. Kripke’s theory of ridged designators argue that that if two things are identical, they must be identical in all possible worlds, meaning it is metaphysically impossible for them to be separate.

We know that in this world water is H₂O, but it seems that we can conceive of water without H₂O. It seems that we can imagine a possible world in which there is a substance with the same properties as water yet is not H₂O.

However, Kripke claims that if we conceive of water without H₂O, what we are conceiving of would not be water because H₂O is the essential property of water. An essential property is one which cannot be removed without altering the concept or definition of a thing. If water is identical to H₂O, there cannot be a possibility that it isn’t H₂O, as that would lead to the absurd result that something could possibly not be what it is.

Applying this to the zombie argument. If phenomenal properties are identical to physical properties, then (like with water and H₂O) they must be identical in all possible worlds. If physicalism is true, there is no possible world in which a physical duplicate of a conscious person could exist without consciousness; a zombie world is not possible.

Integration: this issue attacks P2 for begging the question. So again, the zombie argument presupposes the truth of its conclusion (physicalism being false) for its premise that what is conceivable is possible (P2) to be true.

Property dualists can respond that water and consciousness are crucially different. Water is indeed the sort of thing which has a chemical formula as its essential property, but phenomenal properties have qualia or what it is like to experience them as their essential property, not some physical property. If this is correct, that phenomenal properties do not have physical properties as their essential property, then their relation to the physical is not one of identity. The physical could exist without the phenomenal, so zombies are possible and the conclusion that physicalism is false is maintained.

However, what is the justification for claiming that the essential property of phenomenal properties is qualia, not something physical? To claim that the phenomenal character of experience is the essential property of consciousness is arguably to presuppose some form of dualism to be true, or to think that how the mind seems to itself is how it really is, like Descartes thought. From a first-person perspective it does seem like qualia are an essential property of consciousness, but we can’t just assume that to be correct. If qualia are identical with some physical property and Kripke is correct to think that identities hold in all possible worlds, then a zombie world is not possible.

Integration: the defence from property dualists begs the question. Their response has to assume that the essential property of consciousness is qualia rather than something physical, which is to assume that dualism is true.

The issue that metaphysical possibility tells us nothing about our world

Physicalists could argue that what is metaphysically possible tells us nothing about reality. This requires Physicalism to restrict what it claims to what is physically possible in this world, i.e., that in this world any physical duplicate must share all its properties. It could then be open to there being possible worlds in which physicalism is false and others in which it is true. So Physicalism could attempt to accept that there is a possible world in which a physical duplicate does not have phenomenal properties, even though that would be impossible in this world. So Physicalism only claims to be true in this world, whereas there are possible worlds where property dualism is true.

Integration: this issue attacks P3, that the metaphysical possibility of zombies means that phenomenal properties of consciousness are not reducible to nor supervenient on physical properties.

However, this cannot be correct under Kripke’s theory of ridged designators. Nothing can be what it is not. If Physicalism is true then the essential property of consciousness is something physical, as of yet not completely discovered, much like how once the chemical formula of water had not been discovered. In that case, consciousness cannot something else, i.e., non-physical.

If dualism is true, then the essential property of consciousness is some non-physical property that does not reduce to nor supervene on physical properties. In that case, consciousness cannot be something else, i.e., physical.

Whatever it is, consciousness has to be what it is. Physicalism can’t claim that consciousness could be or be supervenient on one thing (physical) in one world (like this world) yet non-physical in a another possible world.

Integration: Physicalists therefore cannot restrict their claim to what is physically possible in this world. They have to claim that consciousness is identical to or supervenient on physical properties in all possible worlds, otherwise they would be saying that it is metaphysically possible for something to not be what it is. They would be saying that it’s metaphysically possible for something physical to be non-physical. In that case, physicalists cannot deny P3. Physicalists have to deny P1, the conceivability of zombies or C1, the metaphysical possibility of zombies.

The ‘knowledge/Mary’ argument for property dualism

Frank Jackson created the Knowledge argument to argue against physicalism. Jackson invites us to consider the case of a neuroscientist called Mary who has lived her life in a black and white room during which she has gained a complete knowledge of the physical. One day she leaves the room and sees a red object. Jackson asks the question: ‘does she learn something new?’ – in other words, does she learn what it is like to perceive red objects?

If she learns something new then her previous knowledge was incomplete. Since her previous knowledge was a complete knowledge of the physical, it follows from her learning something new that there is more to know about the mental than just the physical. Therefore, there is more in existence than just the physical, so physicalism is false.

P1. A person who has never experienced the phenomenal property of red could have learned all the physical facts about colour perception.
P2. If they then experience red for the first time, they gain new knowledge, i.e., of what it is like to see the colour red.
C1. Therefore, knowledge about phenomenal properties is knowledge about something non-physical.
C2. Therefore, phenomenal properties are non-physical, i.e., they are neither reducible to nor supervenient on physical properties.
C3. Therefore, property dualism is true.

The ability knowledge response

There is more than one kind of knowledge. Propositional knowledge is knowledge of facts whereas ability knowledge is not. For example, knowing how to ride a bike is not knowledge of a fact. Only propositional knowledge captures facts about the world. Jackson’s argument requires not just that Mary learns something new, but also that what she learns is propositional knowledge in order to justify the conclusion that there are facts which cannot be reduced to nor explained in terms of supervenience on physical facts.

Integration: This issue rejects the inference from P2 to C1. It accepts P2, that Mary gains new knowledge. However, to claim that this knowledge must be about something non-physical (C1), that knowledge must be propositional.

The ability knowledge response suggests that when Mary learns what it is like to see red, she only learns the ability to imagine or recognize the colour red. In that case, there are no facts that Mary didn’t know and so her previous knowledge hasn’t been proven (factually) incomplete and thus the knowledge argument fails.

However, while there may indeed be ability knowledge involved, arguably there is still propositional knowledge. Mary learns not just the ability to imagine red, but also knowledge of what having that ability is like. She could then ask whether the ability of others to see red involves the same qualia as her ability. That is propositional knowledge about ability knowledge.

The acquaintance knowledge response

There is more than one kind of knowledge. Propositional knowledge is knowledge of facts whereas acquaintance knowledge is not. Acquaintance knowledge involves knowing of a person or place through direct contact in experience. For example, acquaintance knowledge of London is not knowledge of a fact. Only propositional knowledge captures facts about the world. Jackson’s argument requires not just that Mary learns something new, but also that what she learns is propositional knowledge in order to justify the conclusion that there are facts which cannot be reduced to nor explained in terms of supervenience on physical facts.

The acquaintance knowledge response suggests that when Mary learns what it is like to see red, she only becomes acquainted with the colour red through direct contact in experience. In that case, there are no facts that Mary didn’t know and so her previous knowledge hasn’t been proven (factually) incomplete and thus the knowledge argument fails.

The issue argues that Mary initially had only propositional knowledge about all physical facts (propositional knowledge about colour perception – neurons firing, etc), and that when Mary leaves the room, she gains new acquaintance knowledge as she becomes acquainted with the process of colour perception when she undergoes such process within her own brain.

We could defend the Knowledge argument by pointing out that acquaintance knowledge can involve propositional knowledge. For example, in addition to acquaintance knowledge of red, Mary also gained new propositional knowledge such as knowing that ‘this is what red looks like’ is true and knowing that ‘other people experience this when they see red’ is true.

However, arguably this response begs the question. by assuming the truth of their conclusion (that some mental properties are non-physical) to show the truth of the conclusion.

Propositions such as ‘this is what red looks like’ and ‘this is what other people experience when they see red’ make use of the demonstrative pronoun ‘this’ which is being used to refer to a phenomenal property. If the phenomenal property of red reduces to or supervenes on the physical (i.e., if mind-brain type identity theory is true) then such propositions are about physical facts and thus cannot be used to show that there are non-physical facts.

More crucially though, the issue rejects that the response’s inference that gaining new propositional knowledge, entails that propositional knowledge must be in a different category because multiple propositions can refer to the same thing in the world which means multiple propositions can refer to the same physical fact. Therefore, such an inference about new propositions to non-physical proposition does not follow.

The New Knowledge/Old Fact response

According to this objection, Mary does not learn new facts, she apprehends a fact she already knew but in a new way. Phenomenal concepts are those which your imagination derives from your experience (similar to Hume’s impressions/ideas, see epistemology notes). Physical concepts are those informed by science (i.e. wavelengths of light, neurons, light sensitive cells, etc).

It is possible in the case of red for both the phenomenal concept of red and the physical concept of red to be two different ways of conceptualizing the same thing. Just as it’s possible for the phenomenal concept of water and the physical concept of H₂O to refer to the same thing. The new knowledge/old fact argument claims that Mary had a physical concept of red but gained the phenomenal concept of red which was a new concept that nonetheless was of the same old fact.

D. Papineau claims that while our brains are only capable of forming a phenomenal concept from sense experience, it is theoretically possible for an organism to exist with the ability to imagine phenomenal concepts like colours that it has not sensed.

This is strengthened by Dennett’s argument that Jackson has snuck in the idea of being omniscient about physical facts as if it were possible for us to understand what that would involve. Perhaps the kind of mind capable of knowing all physical things would be the sort of being Papineau mentioned.