1. Meditation I: On what can be called into doubt
Descartes
Meditation I

2. René Descartes
The Father of Modern Philosophy

3. Rene Descartes (1596–1650)
Born in La Haye en Touraine, a small town in central France, which has since been renamed after him
Introduced Cartesian geometry; through his laws of refraction, he developed an empirical understanding of rainbows; proposed a naturalistic account of the formation of the solar system…
His works were put on the Index of Prohibited Books.
Never married, but he did have a daughter, Francine, born in the Netherlands in 1635 (Francine’s mother was a maid in the home where he was staying).
He lived in the Netherlands for more than 20 years but died in Stockholm, Sweden, on February 11, 1650.

4. Descartes took a job tutoring Queen Christina of Sweden
Descartes took a job tutoring Queen Christina of Sweden. It didn’t work out.

5. Descartes’ Milieu: The Scientific Revolution
Against Ptolemeic astronomy
Copernicus ( ), Galileo ( ), Kepler ( ), Newton ( )
Mathematization
‘Philosophy [i.e., physics] is written in this grand book—I mean the universe— which stands continually open to our gaze, but it cannot be understood unless one first learns to comprehend the language and interpret the characters in which it is written. It is written in the language of mathematics’ (Galileo)
The mechanical universe: against Aristotelian teleology of ‘substantial forms’ (‘final cause’)

6. Aristotle’s Universe

7. A Scientific Revolution: Ptolemy  Copernicus

8. Descartes Anti-Scholastic Program
Against explanation by reference to ‘substantial forms’: reject teleology (‘final cause’) as explanatory principle in favor of mechanistic explanation.
Teleology assumes mentality: stone doesn’t intend to fall to earth as a goal etc.
Against empiricism: Scholastics’ method was prone to doubt given their reliance on sensation as the source for all knowledge (from Aristotle’s empiricism)
Ideas of sensation aren’t ‘clear and distinct’
Continental Rationalists vs. British Empiricists

9. What can we know…and what is knowledge???
The JTB Account: Knowledge as justified true belief
What is justification?
What is truth?
What is belief?
Plato give a working definition of knowledge. So we have to ask: What’s justification? What’s truth? And what’s belief?

10. What is truth?

11. A propositional attitude
Belief
A propositional attitude

12. Belief
We call beliefs ‘true’ or ‘false’ in virtue of the truth value of the propositions believed.
By ‘belief’ we don’t mean ‘mere belief’
Believing doesn’t make it so - denial doesn’t make it not so.
We may believe with different degrees of conviction.

13. Having good reasons for what you believe
Justification
Having good reasons for what you believe

14. Foundationalism
Some beliefs are properly basic and that the rest of one’s beliefs inherit their epistemic status (knowledge or justification) in virtue of receiving proper support from the basic beliefs.
Foundationalists have two main projects:
a theory of proper basicality (that is, a theory of noninferential justification) and
a theory of appropriate support (that is, a theory of inferential justification).
The most well known foundationalist is Descartes, who takes as the foundation the allegedly indubitable knowledge of his own existence and the content of his ideas.  Every other justified belief must be grounded ultimately in this knowledge.

15. Descartes Quest for Foundations
Clearing the ground: the method of hyperbolical doubt
‘preconceived opinions’ must be ‘set aside,” says Descartes, “in order to lay the first foundations of philosophy’ (1643 letter to Voetius, AT 8b:37).
The central insight of foundationalism is to organize knowledge in the manner of a well-structured, architectural edifice.
Foundation of unshakable first principles
Superstructure of further propositions anchored to the foundation via unshakable inference
Compare Euclid’s geometry

16. Descartes’ Methodological Doubt
I will apply myself earnestly and freely to the general overthrow of all my former ideas
None of these things are certain:
Empirical facts
Truths of mathematics
The existence of God

17. Methodological Doubt: The Program
‘I realized that if I wanted to establish anything in the sciences that was stable and likely to last, I needed—just once in my life—to demolish everything completely and start again from the foundations.’
Method: withhold assent from any propositions that aren’t certain and indubitable.
We’ll consider beliefs by category:
Beliefs about ordinary objects gotten via sense experience
Mathematical propositions
Theological doctrine
Arguments aimed to show that none of these are certain…
The Argument from Sensory Illusion
The Dream Argument
The Evil Demon Argument

18. Sense Experience ‘Ordinary objects’ ‘Sense Experience’
Veridical Experience: experience of real things as they really are
Non-veridical: illusions, mirages, dreams, etc.
Subject to methodological doubt: ‘Whatever I have accepted until now as most true has come to me through my senses. But occasionally I have found that they have deceived me…
Experience and judgement: can I avoid being mistaken under normal conditions if I’m careful, and not insane?

19. Argument from Sensory Illusion
Our senses deceive us some of the time.
Whatever can happens some of the time, possibly can happen all of the time.
Therefore, It’s possible that our senses can deceive us all of the time.

20. Things are not always as they seem

21. Experience and judgement: Can we avoid being fooled?
Müller-Lyer Illusion (interactive)

22. Parallel lines?
The Café Wall Illusion in motion

23. What’s the illusion here?

26. Sense experience and judgement: expectation

27. We expect to see faces

28. We expect to see faces
Face on Mars, Face in beans, ...and more faces

29. Ambiguity Illusions

30. Figure-ground ambiguity

31. Rubin vase

32. More Rubin Illusions

33. And this

34. Autokinetic effect: illusions of motion
UFOs
Stepping Feet

35. Totally mad illusions!

36. Close up

37. The Greek version

39. Some dreams are so vivid that they seem real.
Therefore, any waking experience could be a vivid dream.

40. Sense Experience: Dreaming
'I am a man, and…I am in the habit of sleeping, and representing to myself in dreams those same things, or even sometimes others less probable, which the insane think are presented to them in their waking moments…At the present moment, however, I certainly look upon this paper with eyes wide awake; the head which I now move is not asleep…I cannot forget that, at other times I have been deceived in sleep….and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished; and in amazement I almost persuade myself that I am now dreaming.’
I realize that there is never any reliable way of distinguishing being awake from being asleep. This discovery makes me feel dizzy, [joke:] which itself reinforces the notion that I may be asleep!

41. Deep skepticism Russell’s 5 minute hypothesis
The Brain-in-the-Vat hypothesis
Descartes Evil Demon

42. It was at that precise moment that Stanley realized that he may very well be a brain in a vat.

43. Solipsism The idea that ‘I alone exist’ and create all of reality
Since I realized I created my own reality in every way, I must therefore admit that, in essence I was the only person alive in my universe.
The idea that ‘I alone exist’ and create all of reality
Should we prefer solipcism to the external world hypothesis?

44. Could all this be an illusion?

45. Could all of our experiences be non-veridical?
I’m happy…what’s the problem?

46. Mathematics: More Certain?
So it seems reasonable to conclude that physics, astronomy, medicine, and all other sciences dealing with things that have complex structures are doubtful; while arithmetic, geometry and other studies of the simplest and most general things—whether they really exist in nature or not—contain something certain and indubitable. For whether I am awake or asleep, two plus three makes five, and a square has only four sides. It seems impossible to suspect that such obvious truths might be false.
Empirical sciences vs. math
A posteriori (empirical) and a priori A posteriori (empirical) knowledge is based on experience, on observation of how things are in the world of changing things. A priori knowledge is based on reasoning rather than observation.

47. Math not certain…Descartes rests his case

48. Will Theology Help?
You might say: God would not let me be deceived like that, because he is said to be supremely good.
But, I reply, if God’s goodness would stop him from letting me be deceived all the time, you would expect it to stop him from allowing me to be deceived even occasionally; yet clearly I sometimes am deceived.
And, I don’t (as yet) have any compelling argument for belief in God ‘Let us grant them—for purposes of argument—that there is no God, and theology is fiction. On their view, then, I am a product of fate or chance or a long chain of causes and effects. But the less powerful they make my original cause, the more likely it is that I am so imperfect as to be deceived all the time’.

49. The Evil Demon
Program of methodological doubt: I want to know what I can know on the worst case scenario, i.e. deceiving evil demon.
‘I shall suppose that some malicious, powerful, cunning demon has done all he can to deceive me—rather than this being done by God, who is supremely good and the source of truth. I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely dreams that the demon has contrived as traps for my judgment. I shall consider myself as having no hands or eyes, or flesh, or blood or senses, but as having falsely believed that I had all these things. I shall stubbornly persist in this train of thought; and even if I can’t learn any truth, I shall at least do what I can do, which is to be on my guard against accepting any falsehoods, so that the deceiver—however powerful and cunning he may be—will be unable to affect me in the slightest. ‘

50. The Demon Argument
It's possible that an evil demon exists that deceives me about the existence of an external world and the truths of mathematics (a priori inferences in general).
Anything that can deceive is not a reliable source of knowledge.
Therefore, a priori inferences and the belief in the external world are not reliable.

51. Can the Demon be Defeated?
Given the possibility that our experiences are produced by Descartes Demon (or Berkeley’s God or Dennett’s team of scientists or machines operating the Matrix) can we have any reason to believe that our commonsense and scientific beliefs about the physical world are true??? Or our beliefs about mathematics? Or our theological beliefs?