1. Philosophy Through the Centuries
Descartes
Philosophy Through the Centuries
BRENT SILBY
Unlimited (UPT)

2. Descartes
Smart man. He did not publish some of his work because he thought he would be put in prison for it.
Rough times… The church made all the rules
* The church told people what to think and how to think it. * The church was authority and people were expected to believe everything they said.

3. Descartes
Descartes was interested in “Epistemology”--the study of knowledge. He decided that there are two types of knowledge
1) A posteriori (empirical) knowledge--through observation e.g. that the moon appears in the sky
2) Apriori knowledge--through thought and reason e.g. 2+2=4, or x=x

4. Descartes asked: “How do we know what we think we know. ”
Descartes asked: “How do we know what we think we know?” This is a fundamental question.
He decided that we don’t really know what we think we know. This leads to “skepticism”. 4

5. Without looking, how do you know that it is not raining?
You could infer it from other facts:
e.g. 1. It was not raining 10 minutes ago 2. There were no clouds in the sky 10 mins ago 3. It takes at least 10 mins for rain clouds to form Therefore. It is not raining
From knowing 1,2,3 we can infer the conclusion. But how do we KNOW 1, 2, 3? These are inferred from more basic ideas,
For example…[next page] 5

6. It takes at least 10 mins for rain clouds to form
We know this by referring to our knowledge of molecular activity and air currents.
This knowledge can be reduced to more basic facts, for example activity at the atomic level.
Atomic theory is reducible to even more basic facts, for example sub-atomic facts from quantum theory.
This is built from knowledge in superstring theory.
How do we know superstring theory is accurate?

7. Descartes argued that there needs to be a single most basic idea that underpins all knowledge. He also argued that this single idea needs to be CERTAIN.
Descartes decided to figure out what he could know with absolute certainty. He wanted to to discover the single most basic piece of knowledge.

8. THE PROCESS OF SYSTEMATIC DOUBT
Descartes listed everything he thought he knew. Then for each item on the list he asked, “can I doubt this?”. If the answer was “yes”, he would throw out that item of knowledge.
First he checked knowledge that he got from his sense perceptions…

9. THE PROCESS OF SYSTEMATIC DOUBT
He thought about how he knows he is sitting by a fire… He gets that knowledge from his senses--his eyes etc.
But wait! Sometimes the senses can fool us. I could be dreaming that
I am sitting by the fire, and it seems real to me but I am actually in bed
nowhere near a fire.

10. THE PROCESS OF SYSTEMATIC DOUBT
Or I could be hallucinating or on drugs or something…

11. If it is possible that my senses can fool me (even if they have only done it once), then I can not fully trust them.
Therefore, I cannot really KNOW anything that my senses tell me.

12. Descartes thought that he could get false sense impressions in a variety of ways.
1. Hallucinations
2. He could be dreaming
3. He could be a disembodied soul being fed fabricated sense data about the world by an evil demon.
Brain in Vat, modern version.
[Draw on board]

13. -- Rene Descartes from the “Meditations”, 1641
“I will suppose, then, not that there is a supremely good God, the source of truth;
but that there is an evil spirit, who is supremely powerful and intelligent, and does his
utmost to deceive me. I will suppose that sky, air, earth, colors, shapes, sounds and all
external objects are mere delusive dreams, by means of which he lays snares for my credulity. I will consider myself as having no hands, no eyes, no flesh, no blood,
no senses, but just having a false belief that I have all these things …
I suppose, therefore, that whatever things I see are illusions;
I believe that none of the things my lying memory represents to have happened really
did so;
I have no senses;
body, shape, extension, motion, place are chimeras.
What then is true?
Perhaps only this one thing, that nothing is certain.”
-- Rene Descartes from the “Meditations”, 1641

14. So is there anything that can’t be questioned?
Can we really know anything at all?
Descartes went further. He asked “How can I know that I exist?”
For Descartes to convince himself that the world doesn’t exist, he must exist. He has to exist to be able to convince himself of all this.
Descartes wrote:
If there is an evil demon who always deceives me, then I must exist. If I didn’t exist, then the demon could not deceive me. I need to exist in order to be deceived.

15. The demon will never be able to trick me into thinking that I do not exist. In order to doubt my existence, I have to think. And to think, I need to exist.
But, what is this “I” that thinks and doubts. Am I a body?
No! A body is a physical thing like a tree or chair. And the demon could delude me into thinking falsely that these things exist.
Quote: next slide

16. The demon must have someone to deceive; else there is no deception.
“At this point I come to the fact that there is consciousness; of this and this only I
cannot not be deprived.
I am, I exist; that is certain.
For how long? For as long as I am experiencing…I know for certain that I am, and that at the same time it is possible that all these images, and in general everything of the nature of body, are mere dreams…
What then am I? A conscious being. What is that? A being that doubts, understands, asserts, denies, is willing, is unwilling; further, that has sense and imagination…Even if I am all the while asleep; even if my creator does all he can to deceive me; how can any of these things be less of a fact than my existence?”
The demon must have someone to deceive; else there is no
deception.

17. So what is the thing being deceived? Me, not a body, but a mind.
Its my mind that is being deceived. And my mind is a thinking thing. And in order to think, I must exist.
I am thinking therefore I exist
I think, therefore I am!
Cogito Ergo Sum.

18. DESCARTES IDEA OF BEING HUMAN
Generally, philosophers fall into 3 camps:
1. Materialists/physicalists * Everything in the universe is made of matter and energy in the space time continuum
2. Dualists * Many things are made of matter, but the mind is not
made of matter. The mind is made of ‘mental’ stuff
3. Idealists * Everything is made of mental stuff. There is no physical reality

19. Descartes was a dualist.
He had 3 arguments for dualism.
Argument 1
I can doubt that my body exists
2. I cannot doubt that my mind exists
3. Therefore my mind is not identical to my body

20. Leibniz law is important for the next argument:
“if 2 things are identical, then they must have ALL the same
properties”
Every material thing is extended in space
2. My mind is not extended in space (I can think about an object,
and I can also think about my thinking about that object)
3. Therefore my mind can’t be a material thing because it doesn’t
have a property that material things have

21. Argument 3:
I’m such that I MIGHT survive the death (destruction) of my body
2. My body can’t survive its own destruction
3. Therefore, by Leibniz law, my mind is not identical to my body.

22. Powerpoint by BRENT SILBY
Produced at Unlimited Paenga Tawhiti Christchurch, New Zealand