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  Descartes' Evil Demon Hypothesis The reason
that things seem to you now the way that they do
is because there is an evil demon deceiving you
and manipulating your mind at every second. He
is making you think about certain shapes and
colours and sights and sounds, even though all of
it is an elaborate trick, and none of your
sensations are verifiable. So, even though it
may seem to you now that you are reading typed
words on a screen, you are really just being fed
these sensations by an Evil Demon, who doesn't
want you to know what the world is really like.
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René Descartes (1596-1650)
In his Meditations, Rene Descartes sets out to
determine,
what sorts of things he knows and how he knows
them.
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In order to determine this, he puts forward the
following two methods of doubt

• He decided to examine
• his foundational beliefs--e.g.,
• whether he could trust his
• sensory beliefs, mathematical
• beliefs, etc.
• (ii) If there is even the
• slightest possibility of a
• belief being false, he will
• reject the belief as a
• candidate for knowledge.

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• So, Descartes is
• trying to determine what
• sorts of things he
• knows given that his
• criteria for knowledge is
• infallibility.
• If there is any way that
• he could be mistaken
• about something, he
• will discount it as
• knowledge.

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Descartes applies his method in the following
three ways
  1.The Senses Sometimes Deceive Descartes
reflects on the fact that our senses sometimes
deceive us. When we are looking at something very
small or far away, for example, we can often be
mistaken about the size or shape of the thing in
question. In addition to these kinds of
phenomena, there are also hallucinations, optical
illusions, and after-images. E.g., a straight
stick might look bent when sticking halfway out
of water and a red-orange after-image can appear
for awhile in your field of vision if you've just
stared at the sun too long.
Sources Rene Descartes, Meditations on First
Philosophy, Third Edition, Translated by Donald
A. Cress.
Page Last Updated Oct. 19, 2005 To Meg's
Teaching Page T Meg's Main Page
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Can you trust your senses?
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Descartes admitted that even though the senses
can sometimes deceive in these ways, they are
usually pretty reliable. So at this point he
thought that we should rule out knowledge through
our senses only when it came to things that are
very small or far away, or under certain abnormal
conditions. This still leaves us with much that
we do know (e.g., that there is a screen in front
of you, that you have hands, etc.)
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You Could Be Dreaming!
2. Descartes next questions whether we can
distinguish dreaming from being awake. For
example, in your dreams you are usually quite
convinced that you are not dreaming. No matter
how crazy your dreams may be (e.g., you may fly
or breath under water in your dreams). So how,
Descartes asks, could you possibly be able to
determine whether you are dreaming or awake right
now?
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ARE YOU AWAKE????

• Thus, since it is possible that you may be
  dreaming right now, Descartes is going to
  discount many of our beliefs as candidates for
  knowledge. For example, you do not know that
  there is a screen in front of you, you do not
  know that you have hands, etc. For if you were in
  fact dreaming right now, then none of these
  things would be true. However, Descartes allows
  that perhaps you still know about colours and
  math and logic, since these will remain unchanged
  whether you are dreaming or not.

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Lastly, Descartes entertains the possibility that
he is being deceived by an evil demon. This evil
demon could deceive him into thinking just about
anything--e.g., that 224 even if in fact it
didn't that red is a particular colour even if
it weren't
THE EVIL DEMON
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To illustrate you can imagine that every time
you try counting the sides of a triangle, the
evil demon makes you think there are only three
sides when in fact there are, say, four. All
Descartes really needs is that the evil demon
could get you to do this once or twice, and your
foundation for mathematical knowledge will be
destroyed.
For remember that Descartes' criteria for
knowledge is infallibility so if you are wrong
about something once, then you could be wrong
again, and so you cannot be counted as knowing it.
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I THINK THEREFORE I AM

• Descartes concludes that
• there is only one thing that he
• knows for certain. This is his
• famous cogito ego sum, which is
• roughly translated as "I think,
• therefore I am.
• For Descartes, a thinking thing is "a
• thing that doubts, understands,
• affirms, denies, is willing, is unwilling,
• and also imagines and has sensory
• perceptions." Since the demon would
• have to deceive something in order to
• carry out his deception, the deceived
• must exist

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The Cogito

• But what then am I? A thing that thinks. What is
  that? A thing that doubts, understands, affirms,
  denies, wills, refuses, and that also imagines
  and senses Is it not the very same I who now
  doubts almost everything, who nevertheless
  understands something, who affirms that this one
  thing is true, who denies other things, who
  desires to know more, who wishes not to be
  deceived, who imagines many things even against
  my will, who also notices many things that appear
  to come from the senses?

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