Descartes and the Mind

From: Harnad, Stevan (
Date: Tue Oct 17 1995 - 16:54:26 BST


It is fashionable these days to blame dualism and the mind/body problem
on Descartes. But since religion and belief in an immaterial soul began
much earlier than the 17th century, it all probably has much deeper
roots than that.

But what is Descartes' "cogito," and what does it have to do with

For more about Descrates, go to the Web with Netscape and open:
and also

Rene Descartes (1596 - 1650) was a French philosopher who applied
what he called the "method of doubt" to everything he thought he knew:
He would see whether he could be certain about it, and if not, he set it
aside, to see whether there was something else he could be certain
about: perhaps there would be nothing at all.

First, though, the idea of "certainty" has to be cashed in: One can't
reject things as NOT certain unless one has a model of what IS certain,
and Descartes did have such a model: It was the truths of mathematics
and logic. Those truths were not open to doubt, because they were true
"on pain of contradiction." That means, it was self-contradictory to
suppose them to be false. And of course a contradiction makes no sense
at all; it is impossible. Even in asking a question such as "Is this
certain or is it open to doubt?", no matter what the answer is, it must
not be self-contradictory, or else it is just gibberish.

Supposing the answer was: "THIS is certain." (It doesn't matter what
"this" is for this example.) If besides "This is certain" being TRUE,
it were also NOT TRUE, in other words, if one stated both that it is
TRUE that this is certain and it is NOT TRUE that this is certain, it
should be obvious that one was saying nothing at all, and that there
was no more point asking what was or was not certain.

So Descartes' method of doubt was based on certainty about one thing, to
begin with, and that is that a contradiction is impossible: That ALL
statements of the form "This is TRUE and this is NOT TRUE" are NOT TRUE!

And the reason Descartes knew he could not doubt the truths of
mathematics and logic is that those truths are all based on proofs
that, if they were NOT true, would lead to a contradiction (so they
must be true). Even 1 + 1 = 2 is like that. Arithmetic is based on
axioms, which we assume to be true. Our assumptions could be false, but
that does not matter, because when we do a proof it is always
CONDITIONAL on the truth of the axioms: In mathematics, I never prove
that something is true "no matter what"; I only prove that it's true IF
the axioms are true. In fact, the longhand version of a proof, if it
starts right from the axioms, is always of the form: If the axioms were
true and this were false, that would create a contradiction (so, this
must be true!).

Anyway, enough about mathematics and contradictions. You all see what I
mean. We wouldn't get far if we didn't exclude contradictions. I
would tell you something (e.g., that Descartes was born in 1596) and I
would also tell you the opposite (that Descartes was NOT born in 1596).
That would be nonsense; the only way to make sense of the statement that
Descartes was born in 1596 is to conclude that IF it is true, then it is
CERTAIN that the statement that Descartes was NOT born in 1596 is false.

That's what Descartes meant by certainty. It was there in maths and
logic, clearly. Was anything else certain, not open to doubt?

He moved from maths to science: Can I be certain that the "laws" of
science are true? Copernicus, a hundred years before Descartes, had
discovered laws of planetary motion. They seemed to be true in every
case examined. Could we be certain they were true? Descartes noticed that
one could not, because it is always possible that in the next case they
will fail. Scientific laws are only PROBABLE; they are not certain.
All the evidence seems to support them, but evidence cannot PROVE them.
It is not self-contradictory that future evidence should fail to support

So scientific truth does not survive the test of doubt (even though
Descartes hoped it would, and kept trying to save them!). What about
everyday things, such as the fact that there are animals and trees and
chairs and tables out there? Descartes found he could doubt those too:
Maybe it's all just a dream or an illusion. It SEEMS as if there is an
outside world, but it is certainly not self-contradictory that there
should be no outside world at all, and that it should all be a figment
of Descartes' imagination.

What's left then? Science is gone. The outside world, and all its
objects and people (including your own BODY) are gone. Is there nothing
left that one can be certain about, that is not vulnerable to this
method of doubt?

Descartes did find that there was one thing left that he could not
doubt, and that was the fact that he was doubting! His famous "cogito
ergo sum" (I think, therefore I exist) is meant to express this
insight, but since a lot of the details about what "thinking" is, and
what "I" am, and what "existence" is are clearly still open to doubt, it
is probably better to state his insight in a more theory-independent
way (since theory could always be wrong): Descartes could not doubt
that he was doubting, so he could be certain that "doubting" was going
on (whatever "doubting" might actually be).

Note that this was not just the FORMAL certainty of mathematics and
logic. In mathematics, truths are stated formally, in symbols; it
doesn't matter what the symbols mean, certain things are true about them
merely on the basis of their form: You can be SURE that the statement
"This is true and this is not true" in NOT true, no matter what "this"
is. That's what's meant by a formal truth.

Well Descartes' cogito sounds a bit like a formal truth at first:
"It is true that I am doubting, therefore it is NOT true that I am NOT
doubting." (Here we are just substituting "I am doubting" for "this.")
But this kind of formal noncontradiction is not what the insight behind
Descartes' cogito was based on. It was based on what we MEAN by "I am

For doubting is an EXPERIENCE. It's something that you know is
happening, because it is happening to YOU. You may not be sure what
"you" are, particularly, but that doesn't matter. Even if the
experience only lasted for 10 seconds, and then was gone, and there was
nothing -- no "you" -- before or after it, it would still not be open
to doubt that the experience was going on, while it was going on. The
experience itself would be the evidence -- indeed the "proof" -- that
there was an experience going on. It could be a crazy experience: It
could even be the experience that "I don't exist" that some people with
mental disorders sometimes have. That would not matter; that would not
make the cogito false, any more than intellectual handicaps that may
make someone unable to understand the truths of mathematics -- or even
unable to understand the impossibility of a contradiction -- would make
mathematics or the law of noncontradiction any less certain.

If an experience is indeed going on, then it cannot be doubted that it
is going on (even if the person having the experience doesn't understand
this!). For an experience is of how things SEEM. It is possible that the
outside world, for example, IS NOT the way it seems, but it is not
possible that it does not SEEM the way it SEEMS. Because seeming is just
experiencing. You can doubt what your experience tells you about the
way things ARE (and, in the case of, say, hallucinations, you would be
right!), but, you cannot (if you are in your right mind, rather than
delirious) doubt what your experience tells you about the way things

Here is another example: You have a toothache. Part of that experience
is that you seem to have a tooth, one that hurts. But you could be
wrong. There could be nothing wrong with your tooth, even though you
have a toothache. In fact, you may not even HAVE the tooth. (It could
have been removed: a more tragic version is that of phantom limb pain
in people whose limb has been amputated, so it is no longer there, yet
it still hurts). So having a toothache does not guarantee that there is
something wrong with your tooth, or even that you HAVE a tooth. But
what it does guarantee is that you SEEM to have a tooth; it feels like
you have a tooth, one that is hurting Moreover, it guarantees that you
are hurting (not clear where or why, but you are definitely hurting).

Hurting? or only SEEMING to be hurting? Here the IS/SEEMS difference
breaks down, because hurting itself is an experience. It is not about
something else, which is open to doubt and might not exist. It is just
an experience. The theoretical details could be wrong: You may have a
theory of what "pain" is, and that could be wrong. But that EXPERIENCE
(which, for most of us, is an unpleasant one), of hurting, is not open
to doubt, because no experience is open to doubt AS EXPERIENCE. The
doubt starts only when it comes to what the experience seems to be
saying about the world, about its causes, about the reality behind the
appearances. But as long as you consider only the appearances, the
seeming, the experience, it is not open to doubt.

So where did dualism and the mind/body problem come from? Well, how can
you possibly equate something as certain and immediate as experience with
something as uncertain and remote as a physical substance? How, in other
words, can you give a PHYSICAL explanation of experience?

But I don't think mind/body dualism started or ended with Descartes, so
we will have more to say about this....

There you have it for now. Ruminate on all this for a while, and let me
have your reactions, in quote/comment form. The reason this is
important is because it seems to put something extremely subjective and
psychological (experience) on a par with something that is as objective
and UNpsychological as anything can be: the truths of mathematics.
These are the two things we can be certain about: the truths of
mathematics and the reality of experience. All else is open to doubt.

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