Descartes - Dualism

Descartes on DualismDescartes on DualismStephen LawStephen Law

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Descartes on dualismDescartes on dualism

Descartes is of course a dualist. But what is dualism?

Students often muff this. They say something like “Dualism is the view that mind and body are different”.

In fact the key term to use here is substance. A “substance” is something that is not dependent upon any other thing for its existence.

Descartes is of course a dualist. But what is dualism?

Students often muff this. They say something like “Dualism is the view that mind and body are different”.

In fact the key term to use here is substance. A “substance” is something that is not dependent upon any other thing for its existence.

Distinct substancesDistinct substances Being substances means each can

exist on its own, without the other. So substance dualism entails that minds can exist on their own: disembodied.

Being substances means each can exist on its own, without the other. So substance dualism entails that minds can exist on their own: disembodied.

Oh! my mind has floated off from

my body!

Body

Mind

An Argument for DualismAn Argument for Dualism

Descartes is often portrayed as offering an argument for Dualism called the THE ARGUMENT FROM DOUBT.

Here’s a first example of such an argument…

Descartes is often portrayed as offering an argument for Dualism called the THE ARGUMENT FROM DOUBT.

Here’s a first example of such an argument…

FIRST ARGUMENT FROM DOUBT I don’t doubt I exist I do doubt my body exists Therefore, I am not identical with my

body This argument is an application of

Leibniz’s law: If objects are identical, they share all

the same properties (so, if they don’t share all the same properties, they are not identical).

FIRST ARGUMENT FROM DOUBT I don’t doubt I exist I do doubt my body exists Therefore, I am not identical with my

body This argument is an application of

Leibniz’s law: If objects are identical, they share all

the same properties (so, if they don’t share all the same properties, they are not identical).

Leibniz’s LawLeibniz’s Law

We regularly use this law to show that things are not identical. Suppose you are an explorer and you discover a mountain…

We regularly use this law to show that things are not identical. Suppose you are an explorer and you discover a mountain…

Leibniz’s lawLeibniz’s law

Later you discover another mountain. Only you are not sure if it is a new mountain. Maybe it’s just the first mountain seen from a different angle? How might you show that the mountains are not identical?

Later you discover another mountain. Only you are not sure if it is a new mountain. Maybe it’s just the first mountain seen from a different angle? How might you show that the mountains are not identical?

Leibniz’s lawLeibniz’s law Find a property one mountain has but not the

other. For example, if you show that mountain 1 is 10K feet high and mountain 2 is not 10K feet high, you can then apply Leibniz’s law like so: Mountain 1 is 10K feet high Mountain 2 is not 10K feet high Therefore Mountain 1 is not identical with

mountain 2. This argument is COGENT!

Find a property one mountain has but not the other. For example, if you show that mountain 1 is 10K feet high and mountain 2 is not 10K feet high, you can then apply Leibniz’s law like so: Mountain 1 is 10K feet high Mountain 2 is not 10K feet high Therefore Mountain 1 is not identical with

mountain 2. This argument is COGENT!

Leibniz’s lawLeibniz’s law

Our first argument from doubt has the same form.

It points out that I and my body differ in our properties: one has the property of being something I doubt exists whereas the other does not.

The argument concludes that I am not identical with my body.

Our first argument from doubt has the same form.

It points out that I and my body differ in our properties: one has the property of being something I doubt exists whereas the other does not.

The argument concludes that I am not identical with my body.

Refuting the argument from doubt - masked man fallacyRefuting the argument from doubt - masked man fallacy

Suppose I see a masked man robbing a bank.

Suppose I see a masked man robbing a bank.

Masked man fallacy (cont.)Masked man fallacy (cont.) Later, my father is accused. Can I prove

his innocence like so? I don’t doubt the masked man robbed

them bank I do doubt my father robbed the bank Therefore the masked man is not

identical with my father

Later, my father is accused. Can I prove his innocence like so? I don’t doubt the masked man robbed

them bank I do doubt my father robbed the bank Therefore the masked man is not

identical with my father

Masked man fallacy (cont.)Masked man fallacy (cont.) The property of being someone I doubt

robbed the bank is not a property to which Leibniz’s law applies. True, my father has this property and the robber doesn’t. But that doesn’t entail that my father is not the asked man.

Leibniz’s law does not apply to properties involving someone’s attitude towards a thing (liking, hating, believing, hoping, etc.)

The property of being someone I doubt robbed the bank is not a property to which Leibniz’s law applies. True, my father has this property and the robber doesn’t. But that doesn’t entail that my father is not the asked man.

Leibniz’s law does not apply to properties involving someone’s attitude towards a thing (liking, hating, believing, hoping, etc.)

Masked man fallacyMasked man fallacy This argument from doubt also seems to

commit the fallacy: I don’t doubt I exist I doubt my body exists Therefore I am not identical with my body

The property of being something I doubt exists is a property involving my attitude towards something.

This argument from doubt also seems to commit the fallacy: I don’t doubt I exist I doubt my body exists Therefore I am not identical with my body

The property of being something I doubt exists is a property involving my attitude towards something.

A second argument from doubtA second argument from doubt

What about this argument from doubt:

I CANNOT doubt I exist I CAN doubt my body exists Therefore: I am not identical with my

body

Does this argument far better?

What about this argument from doubt:

I CANNOT doubt I exist I CAN doubt my body exists Therefore: I am not identical with my

body

Does this argument far better?

The second argument refutedThe second argument refuted

Define “wibble”: whoever is actually Bert’s favourite person. I CANNOT doubt I am in pain I CAN doubt wibble is in pain Therefore I am not wibble.

But I could still be wibble. So it seems what I can or cannot doubt also isn’t a property to which Leibniz’s law applies.

Define “wibble”: whoever is actually Bert’s favourite person. I CANNOT doubt I am in pain I CAN doubt wibble is in pain Therefore I am not wibble.

But I could still be wibble. So it seems what I can or cannot doubt also isn’t a property to which Leibniz’s law applies.

Two arguments from doubtTwo arguments from doubt I don’t doubt I exist I doubt my body exists Therefore I am not my body

I CANNOT doubt I exist I CAN doubt my body exists Therefore I am not my body

Summary: Neither argument is valid.

I don’t doubt I exist I doubt my body exists Therefore I am not my body

I CANNOT doubt I exist I CAN doubt my body exists Therefore I am not my body

Summary: Neither argument is valid.

Descartes’ divisibility argumentDescartes’ divisibility argument

My mind is indivisible My body (brain) is divisible Therefore: I am not my body (brain)

This is also an application of Leibniz’s law. Is this argument valid? (reminder - valid

means: necessarily: if the premises are true, so is the conclusion).


This is also an application of Leibniz’s law. Is this argument valid? (reminder - valid

means: necessarily: if the premises are true, so is the conclusion).



Yes it is valid! But that does not mean it’s a good argument.

What might be wrong with it?


Yes it is valid! But that does not mean it’s a good argument.

What might be wrong with it?



The first premise may be question-begging. Your mind might not seem divisible. But if it’s your brain, it is!


The first premise may be question-begging. Your mind might not seem divisible. But if it’s your brain, it is!


The Corpus Callosum case.

This illustrates that while we might not think our minds are divisible, they are!

The Corpus Callosum case.

This illustrates that while we might not think our minds are divisible, they are!

Two different failingsTwo different failings

The divisibility argument is VALID but has a dubious premise.

The two arguments from doubt are INVALID (even if their premises are true).

So the arguments all fail, but for different reasons.

The divisibility argument is VALID but has a dubious premise.

The two arguments from doubt are INVALID (even if their premises are true).

So the arguments all fail, but for different reasons.

Descartes’ legacyDescartes’ legacy

Descartes has been profoundly influential. However, examined more closely, his arguments seem fallacious).

That is not to say there aren’t better arguments for dualism.

Cartesian substance dualism is no longer very popular. However, property dualism remains fairly popular among some philosophers (e.g. David Chalmers).

Descartes has been profoundly influential. However, examined more closely, his arguments seem fallacious).

That is not to say there aren’t better arguments for dualism.

Cartesian substance dualism is no longer very popular. However, property dualism remains fairly popular among some philosophers (e.g. David Chalmers).

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Descartes - Dualism