Overview – Dualism

Dualist theories of the mind argue that there are two kinds of thing – hence, dualism.

The opposite view to dualism is physicalism – the view that there is only one kind of thing: physical.

Substance dualism says there are two kinds of substances: mental substances and physical substances.

Property dualism says there is just one kind of substance – physical – but that physical substances can have two different kinds of property.

Substance dualism

Substance dualism says there are two completely different kinds of substance in our universe:

  • Mental substances
  • Physical substances

Physical substances are things like the trees, cars, houses, etc. Your body – your arms, legs, etc. – is a physical thing as well.

The brain is part of the body, hence it is also physical. But dualists deny that the mind is the same thing as the brain. Instead, dualists argue that the mind is something completely different to the brain – something non-physical.

Most people who believe in souls are dualists. Typically, the soul isn’t thought of as a physical thing – you can’t touch it, or see it, for example. Similarly, things like ghosts and angels are somewhat dualist – they’re not thought to be made of physical stuff.

Descartes’ conceivability argument

Descartes’ conceivability argument can be summarised as:

  • I have an idea of my body as something that is physical and does not think
  • I have an idea of my mind as a thinking thing that is not-physical
  • Therefore, my mind and body can exist separately from each other
  • Therefore, mind and body are two distinct substances

The above characterisation of Descartes’ argument is a bit uncharitable, but this is essentially what he’s arguing.

It does have some intuitive appeal. For example, it certainly seems possible that your mind could exist independently of your body, perhaps as a ghost or something.


What is conceivable may not be possible

Logical, Physical, and Metaphysical Possibility
Venn diagram illustrating different levels of possibility. Something that is logically or metaphysically possible (i.e. something that is conceivable) may be physically impossible (i.e. impossible in our actual world).

Just because we can imagine the mind floating around independently of a body, doesn’t mean this is physically possible.

It might be logically possible – i.e. doesn’t involve a logical contradiction – but just because something is logically possible, this doesn’t automatically mean it is physically possible!

For example, it is not logically possible for a triangle to have 4 sides because it involves a logical contradiction.

But it is logically possible for me to jump on to the moon from earth. It might be physically impossible, but there is no logical contradiction in this idea!

Similarly, just because it’s conceivable/logically possible for a mind to exist independently of a body, this doesn’t automatically mean such a thing is physically possible.

Masked man fallacy

Descartes’ arguments switch from talking about his ideas of mind and body to the actual things themselves. This kind of reasoning is fallacious, as can be shown with the following example:

  • I have an idea of Batman as a caped crusader
  • I have an idea of Bruce Wayne as a billionaire who is not a caped crusader
  • Therefore, Batman is not Bruce Wayne

But Batman is Bruce Wayne, so the conclusion is clearly false. Just because your ideas of Batman and Bruce Wayne are of different people, doesn’t mean they are different people in real life.

Descartes’ conceivability argument uses the same format as the Batman example. However, if this form of argument can lead to false conclusions in the Batman example, it’s possible that Descartes’ conclusion that mind and body are separate is also false.

Basically, just because you have an idea that the two things are separate, it doesn’t mean this is how it is in real life. You can have an idea that Batman and Bruce Wayne are separate people even though they’re not. Similarly, just because you have an idea that the mind and body are separate things it doesn’t automatically mean they are in reality.

Descartes’ divisibility argument

Descartes’ divisibility argument can be summarised as:

    • My body is divisible
    • My mind is not divisible
    • Therefore, my mind and body are separate things

The first premise is definitely true. If you chopped you leg or your arm off, you would be dividing up your body.

The mind, however, does not seem divisible – at least not in the same way. You can’t, for example, have half a thought.


The mind is divisible

There are plenty of cases of mental illness in which the mind does seem to be invisible. For example, someone with multiple personality disorder could be said to have a divided mind.

Another example of this would be people who have literally had their brain cut in half. A corpus callosotomy is a surgical procedure for epilepsy where the main connection between the left and right hemispheres of the brain is severed. Perhaps surprisingly, patients go on to live perfectly normal lives – although there can be a few weird side effects:

“I said, ‘do you believe in God?’, and the right hemisphere went straight to yes. I asked the same question to the left hemisphere […] it went to no right away.”

This suggests that dividing the brain can, in fact, divide the mind.

So, the second premise of Descartes’ argument appears to be false: the mind is divisible.

Problems for Substance dualism

Problem of other minds (and solipsism)

Solipsism is the idea that one’s own mind is the only thing that can be known to exist.

This is the position Descartes ends up at after his three waves of doubt: he doubts the existence of everything – and everyone – except his own mind.

And if substance dualism is true, it seems difficult to avoid these solipsistic doubts.

Each of us only ever experiences our own thoughts, sensations, feelings, etc. We might empathise when we see someone hurt themselves but we don’t literally feel their pain. If we both look at the same sunset we are looking at the same thing but each of us is having a different, private, experience in our mind.

Yet even though you might never literally experience my thoughts, you’d still assume I have them (except maybe in weird philosophical contexts like this one). You don’t seriously doubt whether your friends, family, and random people on the street have minds. You infer from their behaviour that they have a mind that causes their behaviour.

But if substance dualism is true, what grounds do you have to make this assumption? Minds and bodies are two completely separate and independent substances. How do you know there is a mind ‘attached’ to a body? It’s completely possible, on the dualist view, to have physical behaviour without a physical mind. In such a case, what evidence could you possibly find which proves other minds exist at all?

Response: Mill’s argument from analogy

John Stuart Mill gives a ‘common sense’ response to the problem of other minds:

    1. I have a mind
    2. My mind causes my behaviour
    3. Other people have bodies and behave similarly to me in similar situations
    4. By analogy, their behaviour has the same type of cause as my behaviour: a mind
    5. Therefore, other people have minds

However, solipsism can respond that one example of a relationship between mind and behaviour (my own) is not sufficient to prove the relationship holds in all cases. It would be like saying “that dog has 3 legs, therefore all dogs have 3 legs.”

It’s a dubious inference to go from one instance of a relationship (I have a mind that causes my behaviour) to the claim that this relationship holds in all instances (everyone has a mind that causes their behaviour).

Causal interaction

Another issue for substance dualism is how mental things can causally interact with physical things when they are supposed to be two completely separate substances.

Our mental states affect how we behave. If I’m feeling hungry (mental state), it might cause me to move my body (physical thing) to the fridge to get some pizza.

But how does the non-physical mental state transfer over into the physical world and cause things to happen?

Causal interaction problem for substance dualism
How does the non-physical mental state (left) cross over into the physical world (over the red line) and cause changes in my brain that cause me to go and get a pizza?

Note: this is mainly an issue for interactionist dualism, not so much for epiphenomenal dualism.

  • Interactionist dualism: the mind can interact with the physical world (e.g. mental state of hunger causes you to go and get pizza) and the physical world can interact with the mind (e.g. getting hit in the head causes the mental state of pain). In other words, the mental and physical can interact in both directions.
  • Epiphenomenalist dualism: the physical world can cause mental states (e.g. getting hit in the head causes the mental state of pain) but mental states cannot cause changes in the physical world – i.e. the causal interaction is one way. Me going to get pizza would be explained by my brain state, not my mental state. Epiphenomenalism is a subset of property dualism.

It’s easy to explain how physical things interact with one another. When you kick a football, the atoms of your leg (physical thing) connect with the atoms of the ball (also a physical thing) and the ball moves. There’s no mystery here.

But it’s hard to see how a non-physical thing would interact with a physical thing in the same way. It would be like punching a ghost: your hand would go straight through!

This objection was put to Descartes himself by Princess Elisabeth of Bohemia:

    1. Physical things only move if they are pushed
    2. Only something that is physical and can touch the thing that is moved can exert such a force
    3. But the mind is not physical, so it can’t touch the body
    4. Therefore, the mind cannot move the body

We know (4) is false, so there must be a problem elsewhere in the argument.

Of all the premises, (3) seems easiest to dispute. It follows from this that the mind is, in fact, physical. And if the mind is physical then substance dualism is wrong.

Property dualism

Property dualism is the view that there is just one type of substance (physical) but that some physical substances can have non-physical properties.

It differs from physicalism in that property dualists believe a complete description of the physical universe would not be a complete description of the entire universe. In other words, there are facts about our world that cannot be reduced to physical facts.

Specifically, property dualists believe that facts about qualia are non physical. Qualia are the subjective qualities of experience – i.e. what something feels like inside. Examples of qualia include:

    • The redness I experience when I look at a ripe tomato
    • The taste of beer when I have a drink
    • The rough feeling when I run my hand over some sandpaper

Knowledge of qualia is sometimes called phenomenal knowledge – i.e. knowledge of what it is like to have a certain experience.

The zombie argument

A philosophical zombie is a person who is physically and functionally identical to an ordinary human – except they don’t have any qualia.

A zombie might say “ouch!” when it gets stabbed but it doesn’t feel any pain internally.

Such zombies seem conceivable. We can imagine a possible world that is physically identical to this one, with the same people, but without qualia. In this world, you would behave and act in exactly the same way as in the actual world except you’d have no phenomenal experience.

We can use this intuition to form an argument for property dualism similar to Descartes’ conceivability argument:

  • Philosophical zombies are conceivable
  • If philosophical zombies are conceivable then philosophical zombies are metaphysically possible
  • If philosophical zombies are metaphysically possible then phenomenal properties are non-physical
  • If phenomenal properties are non-physical then property dualism is true
  • Therefore, property dualism is true

Responses to the zombie argument

Zombies are not conceivable

Some physicalists argue that qualia (when defined as exclusively phenomenal and subjective properties) don’t really exist. It’s just an illusion.

This illusion is a powerful one. But if we had enough physical knowledge we would be able to understand what we currently call ‘qualia’ in purely physical terms.

If true, then the reason zombies seem conceivable is because we’re labouring under a false illusion that qualia are these spooky non-physical things. Once we understand that qualia are, in fact, just physical things then it becomes inconceivable to imagine a physically identical being that lacks these physical features as this would result in a contradiction. It would be like trying to imagine a married bachelor or a triangle with 4 sides.

Once we understand that qualia = a physical thing, it becomes inconceivable for two physically identical beings not to have identical qualia.

Zombies are not possible

(very similar to the argument above)

Physicalists could plausibly argue that phenomenal properties just are physical properties. They’re the same thing.

Saul Kripke’s definition of identity relations is that if A is identical to B, then A and B are the same thing. It’s not possible in any world for A not to be B as this would result in a logical contradiction.

So, on this definition, if water is H2O, then there is no possible world in which water is not H2O. We can imagine a world where the clear liquid in lakes and rivers is H3O, but then it wouldn’t be water.

Similarly, if phenomenal properties are an essential property of some physical things, then it’s not possible for certain physical things to have different phenomenal properties whilst still being identical. Imagine saying: “phenomenal properties are physical properties but they’re also not physical properties” – it doesn’t make sense (in any possible world).

So, if phenomenal properties are identical to some physical properties, then it’s not logically possible for zombies to exist.

Frank Jackson: the knowledge argument (Mary)

Mary is confined to a black-and-white room, is educated through black-and-white books and lectures relayed on a black-and-white television. In this way she learns everything there is to know about the physical nature of the world. She knows all the physical facts about us and our environment, in a wide sense of ‘physical’ which includes everything in completed physics, chemistry, and neurophysiology, and all there is to know about the causal and relational facts consequent upon all this, including of course functional roles. If physicalism is true, she knows all there is to know.

– Frank Jackson, What Mary Didn’t Know

So, in this thought experiment, Mary has learned all the physical facts about our world from within a black and white room. She knows every possible physical fact – including every physical fact about human experience of colour.

Physicalism argues that everything is physical and so there are only physical facts and knowledge of physical things.

However, Jackson uses this thought experiment to show that Mary doesn’t have all knowledge despite having all physical knowledge. It follows from this that there is such thing as non-physical knowledge. Jackson continues:

It seems, however, that Mary does not know all there is to know. For when she is let out of the black-and-white room or given a color television, she will learn what it is like to see something red, say. […] Hence, physicalism is false.

So, in short, the argument looks something like this:

    • Mary knows all the physical facts about colour
    • Mary does not know what it feels like to see colour
    • Therefore, what it feels like to see colour is not a physical fact
    • Physicalism says that all facts are physical facts
    • Therefore, physicalism is false

Responses to the knowledge argument

Ability hypothesis

Remember from epistemology the three kinds of knowledge:

    • Ability: knowledge how – e.g. “I know how to ride a bike”
    • Acquaintance: knowledge of – e.g. “I know Fred well”
    • Propositional: knowledge that – e.g. “I know that London is the capital of England”

We can accept that Mary learns something new when she leaves the black and white room but reject Jackson’s claim that this new knowledge is non-physical.

Instead, we might argue, Mary gains new ability knowledge.

There is nothing spooky or non-physical about knowing how to ride a bike. The fact that people are able to ride bicycles is not used as an argument against physicalism.

And we can imagine a similar case to the Mary example. However, this time, Mary learns all the physical facts about riding a bicycle (and all the related causal and relational facts) from books and videos, etc. without ever actually touching a bike for herself.

When Mary is given a bicycle for the first time she probably won’t be able to ride it – even though she knows all the physical facts about riding bicycles. This is because knowledge of how to ride a bike isn’t the kind of knowledge you can learn from facts in books. It’s ability knowledge. And ability knowledge is a kind of physical knowledge.

Applied to the original Mary case, some argue when Mary sees red for the first time all she does is gain new abilities. She gains the ability to imagine red, for example. She also gains the ability to distinguish red sensory experiences from green sensory experiences.

Acquaintance hypothesis

Similar to the argument above, we can claim that although Mary learns something new, her new knowledge is still a kind of physical knowledge: knowledge by acquaintance.

Chances are you don’t know Donald Trump personally – even if you know a load of facts about him. Even if you learned every physical fact about Donald Trump you still couldn’t say you know him if you’d never met him.

But Donald Trump’s acquaintances and friends do know him personally. But there’s nothing spooky or non-physical about their knowledge.

You can make a (sort of) similar argument for Mary’s experience of seeing the colour red.

She can know all the physical facts about red – what it is, when people see it, how they react to it, etc. – without being acquainted with redness itself.

Mary is not acquainted with redness because her own brain has never had this property itself. But when she sees red for the first time the property occurs in her brain and she becomes acquainted with redness. Mary gains new knowledge from being acquainted with redness in this way.

New knowledge, old fact

There is more than one way to know the same fact.

For example, “I know there is water in that glass” expresses knowledge of the same underlying physical fact that “I know there is H2O in that glass” does.

But it’s possible to know the former and not know the latter. For example, back before the chemical structure of water was discovered, it is perfectly imaginable that someone could know “there is water in the glass” but not know “there is H2O in the glass”.

The same fact can be understood via two different concepts.

You could argue it’s a similar case with Mary’s knowledge of what it’s like to see red.

Before she left the black and white room, Mary only knew about redness in theoretical terms. But when she leaves and sees red she gains a new concept: the phenomenal concept. And it’s impossible to know what it’s like to see red without this concept.

We can argue that this phenomenal concept just provides a different way of understanding the same underlying fact.

So, Mary doesn’t learn any new, non-physical fact. She just learns a different way of understanding the same fact.

Mary would already know

Another objection would be to reject a main premise of Jackson’s argument: that Mary would learn something new. It’s a similar argument to the qualia don’t exist response to the zombie argument.

We could argue that, if Mary really did know all the physical facts about what it’s like to see red (as the thought experiment claims), then this would include knowledge of what it’s like to see red.

In other words, Mary would already know what it’s like to see red before she left the black and white room.

This claim goes against an intuition most people have. We can’t imagine how Mary could know what it’s like to see red without having seen it herself.

But how can we know that all physical knowledge about seeing red wouldn’t also include knowledge of what it’s like to see red? How can we even imagine what it would be like to have all physical knowledge about something like this?

The intuition that Mary wouldn’t know what it’s like to see red is just that – an intuition. What solid argument is there that all physical knowledge would not also include knowledge of what it’s like to see red?