So my name's Alex Byrne.

I teach philosophy at MIT, and today

I'm going to explain an argument

for so-called mind-body dualism,

the view that we are not physical or material things.

And if we're not physical or material things,

the natural alternative is that we're

mental things of some kind.

Immaterial minds or souls,

as it's sometimes put.

Hence the term "mind-body dualism."

On this view, the universe contains

two quite different sorts of things:

physical bodies like stones

and planets and brains on the one hand,

and non-physical minds on the other.

Well why is this view important?

Well, physical things normally

aren't around forever.

If I smash my watch into tiny pieces

or throw it in a furnace, that's the end

of this beautiful piece of Swiss engineering.

The watch doesn't exist anymore.

Similarly if your body is devoured by worms

or consumed in a crematorium,

that's the end of this beautiful piece

of biological engineering.

Your body doesn't exist anymore.

So, if you're a physical thing,

a complicated bag of cells,

then your eventual bodily destruction

means that there's no hope for immortality.

So, if you're invested in the prospect

of life after death, a lot hangs on

the argument for mind-body dualism.

The seventeenth-century philosopher Rene Descartes

is the most famous proponent

of mind-body dualism, and that's why

the view is sometimes called "Cartesian dualism."

You'll remember Cartesian coordinates

from high school geometry,

and Descartes invented those.

His most famous work is called

"Meditations on First Philosophy,"

which was published in Latin in 1641.

And the sub-title promises that the work

will demonstrate the existence of God

and the immortality of the soul.

We can only do so much in a few minutes,

so we'll have to leave the demonstration

of the existence of God

for another episode of Wi-Phi.

Now the argument I'm going to present

is not quite Descartes's argument

as we find it in the Meditations.

It's basically a variant of Descartes's argument,

given by the contemporary philosopher Saul Kripke

in his classic book "Naming a Necessity,"

which was published in 1980.

And what's more, it's a simplified version

of Kripke's argument.

But even with the simplifications,

I think we can see that it certainly

leads to an argument that deserves

to be taken seriously.

All right, so now to the argument.

Let's give your physical body a name.

Call it "Bert."

Everyone, dualist or not, can agree you and Bert

are intimately connected.

Stamp on Bert's toe, and you feel pain.

If you decide to get some aspirin,

that will result in Bert moving

towards the medicine cabinet.

However, that doesn't mean that you are Bert.

And according to the dualist, you aren't.

There are two things here: you and Bert.

And what the dualist argument tries to establish

is that you are not Bert.

More explicitly, you are not identical to Bert.

You are not one in the same thing as Bert.

Okay, so that's the conclusion.

So now, to prepare for the premises of

the argument, we need a distinction,

between truths that could have been false

and truths that could not have been false.

For example, here's a truth:

I am a philosopher.

That truth could have been false.

I could have been a plumber, say.

Plumbing might have struck me as a more

fulfilling and secure career than philosophy,

and I might have studied

for a plumbing certificate instead

of studying for a PhD in philosophy.

Here's another example:

it's true that there were dinosaurs.

But that could have been false.

Evolution could have failed to produce

any dinosaurs, or life might not have evolved at all.

So some truths, then, could have been false.

But some truths could not have been false.

They had to be true, come what may.

For example, here's a logical truth:

either there were dinosaurs, or there were no dinosaurs.

That's true, but it didn't just happen to be true.

It couldn't have been otherwise.

However the world turned out,

that logical truth would have been true.

Here's another example, which is

the relevant one for our purposes.

Imagine that the President of the United States, say,

is sitting opposite us.

I point to him and say, "He is Barack Obama."

That's true.

But could it have been false?

Well, how could it?

How could that very man fail to be Barack Obama?

We have just one thing here:

that man, also known as "Barack Obama."

When I say "He is Barack Obama,"

I'm picking out the same thing twice over.

It's as if I were to say "He,"

pointing at Obama, "is him," pointing at Obama again.

A thing can't fail to be identical to itself.

So "He," here I point at Obama,

"can't fail to be identical to Obama."

So, when I say "He is Obama," what I say

is not just true, it had to be true.

It's one of those truths like that

logical truth I just mentioned.

It could not have been false.

If you're inclined to doubt this,

you're probably thinking of some different,

but related, truth that could have been false.

For example, it's also true that he,

pointing at Obama, is named "Barack Obama."

But that's a truth that could have been false.

He might have had some, different name say Fred Blogs.

But the truth that he is Barack Obama is not the same

as the truth he's named Barack Obama.

The first truth is not about language,

although of course it is stated in language,

like truths in general.

It's just about the man, Barack Obama.

The second truth is about language,

at least in part.

Specifically, it's about the name "Barack Obama."

And of course these are quite different things.

Barack Obama is the president, but his name

has not been elected to any office.

All right, now we're ready for the argument.

Go back to you and Bert, your body.

Imagine I point to you and say "You are Bert."

Suppose that's true.

Then, since it's just like the Obama example,

it's one of those truths that could not have been false.

In other words, if it's true that you are Bert,

it had to be true that you were Bert.

You are Bert, come what may.

So this gives us the first premise

of our argument for dualism.

If it's true that you are Bert,

then it could not have been false that you are Bert.

But hold on.

Couldn't you have existed without Bert existing?

For example, you can imagine being

disembodied, not having a body at all

or you can imagine that you have

another body, Bertha, not Bert.

Imagining these situations is not at all

like imagining, or trying to imagine, say,

a situation in which there's a round square table.

That situation seems obviously impossible,

not a situation that could have obtained.

There could not have been a round square table.

But there seems nothing at all impossible

about a situation in which you exist without Burt existing,

perhaps because you're disembodied,

perhaps because you have Bertha

and not Bert as your body.

This is not the actual situation,

but it seems like a possible situation.

You could have existed without Bert existing.

But if you could have existed without Bert existing,

then it could have been false that you are Bert.

A situation in which you're around and Bert isn't

is a situation in which you aren't Bert.

So this gives us our second premise,

"it could have been false that you are Bert."

So now notice that the second premise

is the negation of the sentence

after the word "then," in the first premise.

So our two premises have

the following abstract form:

"If P, then Q; and not Q."

And premises of this form logically imply,

by a rule of inference called "modus tollens," "not P."

And our two plausible seeming premises, then, imply

"it's not true that you are Bert."

In other words, you are not Bert,

which is the dualist conclusion.

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