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.

René Descartes (1596-1650)

In his ‘Meditations’, Rene Descartes sets out to determine,

what sorts of things he knows and how he knows them.

(i) He decided to examinehis foundational beliefs--e.g., whether he could trust hissensory beliefs, mathematicalbeliefs, etc.

(ii) If there is even theslightest possibility of abelief being false, he willreject the belief as a candidate for knowledge.

In order to determine this, he puts forward the following two methods of doubt:

So, Descartes istrying to determine whatsorts of things he knows given that hiscriteria for knowledge is

infallibility.

If there is any way that he could be mistaken about something, he will discount it as knowledge.

  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.

Descartes applies his method in the following three ways:

Can you trust your senses?

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.)

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?

You Could Be Dreaming!:

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.

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 2+2=4 even if in fact it didn't; that red is a particular colour even if it weren't;

THE EVIL DEMON

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.

I THINK THEREFORE I AMDescartes 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

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?”