1. Descartes’ First Meditation
Accepting the Skeptic’s Challenge

2. A Note on Descartes’ Method:
Descartes intends to refute the skeptic by showing that at least one of her premises is false.
To do so he will begin by assuming that all of the skeptic’s premises are true.
He will then derive a contradiction. (This is a standard method of proof in mathematics.)
He will use the contradiction to pinpoint the false premise.

3. Descartes’ Goals
Descartes wants to refute the most radical form of skepticism - global skepticism.
In doing so, he wants to simultaneously defend Rationalism - the view that certain knowledge is gained only through pure reasoning.
From his rationalism, Descartes will argue for Foundationalism - the view that certain knowledge is the foundation for all other knowledge.

4. The Method of Doubt
Descartes begins his epistemological investigations by conceding that much he had previously believed turned out to be false and resolving to find some unquestionable truth.
But how does one go about finding such a truth, especially given the risk that one might inadvertently appeal to other falsehoods that one mistakenly believes to be true?

5. Well, you could scrutinize each of your beliefs individually trying to correct any errors, but there are two problems with this approach.
First, it’s very inefficient. How long do you imagine it would take to investigate and scrutinize all of your beliefs, one-by-one?
Second, your examination of any one belief will inevitably rely largely on other beliefs that you accept.

6. For at least this reason, Descartes proposes an alternative method.
In particular, such an examination will rely on principles that you accept concerning the proper way to identify false beliefs and arrive at true beliefs.
But if some or all of those underlying principles are mistaken, then your ability to identify and correct false beliefs will fail.
For at least this reason, Descartes proposes an alternative method. 6

7. Instead of considering each belief one-by-one, Descartes proposes to reject any belief that is possibly false.
This is a radical proposal, since a belief can be possibly false even if it is very unlikely to be false.
Descartes is ruling out any belief that might conceivably be false, and will only accept “completely certain and indubitable” beliefs as foundations for eliminating errors. 7

8. The Method of Doubt We may call this Descartes’ Skeptical Criterion:
It says that, since real knowledge is certainly or indubitably true (i.e., it could not be false), any belief that could be false will be rejected.
In this way, Descartes hopes to determine which, if any, of his beliefs are certainly or indubitably true.
Those beliefs will be immune to skepticism.

9. The Three Stages of Doubt
As we said, Descartes’ method is to assume that the skeptic’s argument is true and derive a contradiction.
The first step in this process is his acceptance of the Skeptical Criterion.
The rest of the process happens in three stages, moving to increasingly radical forms of skepticism with each stage.

10. The Three Stages of Doubt
These stages correspond to different kinds of beliefs and the ways in which they might be mistaken.
In the Third Stage Descartes finds his contradiction (argued for in Meditation II). 10

11. Stage 1: Doubting the Senses
Here, Descartes accepts the skeptic’s premise that the senses deceive us:
“All that up to the present time I have accepted as most true and certain I have learned either from the senses or through the senses; but it is sometimes proved to me that these senses are deceptive, and it is wiser not to trust entirely to any thing by which we have once been deceived.”

12. Argument of Stage 1
Descartes’ argument in Stage 1 is the same as the skeptical argument we’ve already seen:
If some of our perceptual beliefs are erroneous, then it is always possible that any of our perceptual beliefs are erroneous.
If it is always possible that any of our perceptual beliefs are erroneous, then we never know that any of our perceptual beliefs are true.
Some of our perceptual beliefs are erroneous.
Therefore, it is always possible that any of our perceptual beliers are erroneous.
Therefore, we never know that any of our perceptual beliefs are true.

13. A Common Sense Objection
Descartes raises a common sense objection to this argument:
Wouldn’t I be insane if I were to “deny that these hands and this body are mine?”
The common sense point is that I can tell when my senses are deceiving me, and this ability allows me to gain knowledge through my senses.
In other words, common sense rejects premise 2 of the skeptical argument.

14. Stage 2: Dreams.
In response to the Common Sense objection to sensory doubt, Descartes claims that we may not be able to determine when our senses deceive us:
“At the same time I must remember that I am a man, and that consequently I am in the habit of sleeping ... How often has it happened to me that in the night I dreamt that I found myself in this particular place ... whilst in reality I was lying undressed in bed!”

15. Dreaming v. Waking
Have you ever dreamt that you had waken from sleep? Could you tell that you were still dreaming? Are you dreaming right now?
Descartes’ point is that, when dreaming, you lack the ability to know that your senses are deceiving you.
To be able to know when your senses deceive, they must sometimes tell the truth.

16. Dreaming v. Waking
But when you are dreaming, your senses never tell the truth.
Moreover, there is no way to distinguish dreaming from waking, and so no way to know whether or not your sensory experiences are true.
The dream hypothesis strengthens premise 2 against the common sense objection.

17. Another Common Sense Reply
Again, Descartes considers a common sense reply:
“... we must at least confess that the things which are represented to us in sleep are like painted representations which can only have been formed as the counterparts of something real and true, and that in this way those general things ... are not imaginary things, but really existent.”

18. Another Common Sense Reply
The point here is that our senses could not represent something in our dreams that we did not perceive while awake. So in order to have those representations in my dreams, I must have perceived something real.
Descartes extends this point: Notions from science and mathematics need not be false in our dreams.
As he says, 2+2 = 4 “whether I’m awake or asleep.”

19. Stage 3: The Evil Genius
In order to overcome this objection to Stage 2, Descartes moves to Stage 3.
Here, we assume that an omnipotent, omniscient being makes all of your beliefs false - including ‘2+2 =4’.
This stage represents the most radical form of skepticism, and here is where Descartes wants to find his contradiction.

20. Descartes’ evil genius is capable of deceiving his subject about the material world, her own physical nature, and even arithmetic and geometry.
Again, what is important for Descartes isn’t that it’s plausible or probable that such a being exists - but only that it’s possible.
Given the possibility of such a being, Descartes seems to be in a position to reasonably doubt every belief he has - from everyday beliefs about the material world and his own body to abstract beliefs about mathematics.

21. How Could Math Be False?
You might wonder how the evil genius could make a mathematical truth like “2+2=4” false.
The intuition here is that we are all prone to error when making calculations
The evil genius simply causes us to make systematic mistakes every time, without our knowing we’ve made a mistake.
In that case, we’d never know that any of our calculations are true, which is all that’s needed for the skeptical hypothesis.

22. Argument of Stage 3
The argument here has the form of a standard modus ponens.
If it is possible that an Evil Demon deceives me, then it is possible that all of my beliefs are false.
It is possible that an Evil Demon deceives me.
Therefore, it is possible that all of my beliefs are false.

23. The Cogito
Given the evil genius hypothesis, Descartes serious considers the possibility that it is “not within [his] power” to know anything true or that he can only “know for certain that nothing is certain.”
However, Descartes famously claims that, even in the circumstances created by the evil genius hypothesis, he can know at least one thing with certainty - that he himself exists.

24. The Cogito
His famous statement of this piece of certain knowledge - ‘I think, therefore I am’ - is called the cogito (after the Latin version, cogito ergo sum).
Descartes’ point is that it is not possible for him to have any beliefs at all - even false beliefs - if he doesn’t exist.

25. The Cogito
Think about it: If you did not exist, could you be deceived into believing that you do exist?
Descartes thinks that it is self-evident that his existence is a necessary condition for the presence of any of his thoughts, whether or not those thoughts are true.

26. The evil genius cannot, then, deceive him about his own existence.
And so, the moment Descartes asserts his own existence - the moment he thinks he exists - he has a certainly or indubitably true belief.
This is the evidence he needs to show that skepticism is false.
The contradiction he sought is as follows: if all of my beliefs are false, then at least one of my beliefs is true.
And it is evidence that each of us can have for ourselves.

27. Notice that Descartes does not claim that he knows about his physical body and its history, or that he knows about the existence of everyday objects.
What he knows so far is only that he is a “thinking thing; that is, a mind, or intellect, or understanding, or reason.”
If this is all Descartes can know, his knowledge is very limited. After all, the evil genius could continue to deceive him about everything else.