External World Skepticism

1. External World Skepticism

2. For Next Time Read the Feldman selection (94-99)

3. Introduction Intuitively we think we know lots of things: • Knowledge of our immediate environment • Knowledge about the past (I didn’t eat breakfast) and future (the sun will rise tomorrow) • Knowledge of general facts about the world (Africa is a continent) • Knowledge of our own mental states • Knowledge of the mental states of others

4. Introduction Intuitively we think we know lots of things: • Scientific knowledge • Moral knowledge • Religious knowledge • Mathematical knowledge

5. Skepticism A skeptic is someone who denies that we know (or are justified in believing) something.

6. Introduction Skeptics can differ with regard to the domain of knowledge that they deny: • Global Skepticism: The claim that we don’t (and/or can’t) know anything. • Domain Specific Skepticism: The claim that we don’t (and/or can’t) know anything in a particular domain.

7. Introduction There are very few global skeptics.

8. Pyrrho 360-270 BC

9. Introduction • Global skepticism may even be self defeating: • If no one can know anything then the skeptic cannot know that global skepticism is true!

10. Introduction Domain specific skepticism can be far more defensible and interesting. • Skepticism about knowledge of the future • Skepticism about other minds • Skepticism about moral knowledge • Skepticism about scientific knowledge • Skepticism about religious knowledge • Skepticism about the external world

11. External World Skepticism The external world skeptic claims that we cannot know anything about the world outside of our minds.

12. External World Skepticism Mental States • Thoughts • Beliefs • Desires • Current experiences and sensations • Emotions

13. External World Skepticism Things in the External World (everything else): • Plants • Animals • Other people • Planets/galaxies etc.

14. External World Skepticism If the skepticism about the external world can be defended it would undermine a lot of what we take ourselves to know: • Knowledge of our immediate environment • Knowledge about the past • Knowledge about the future • Knowledge of general facts about the world • Knowledge about the mental state of others • Scientific Knowledge • Moral knowledge (?) • Religious knowledge (?)

15. Descartes’ Biographical Information • Rene Descartes (1596-1650) is sometimes called “the father of modern philosophy” • Probably among the the smartest people ever • As a mathematician invented the Cartesian coordinate system

17. Descartes’ Biographical Information • As a scientist he figured prominently in the Scientific Revolution • His Meditations on the First Philosophy is still one of the great philosophical works ever written and is still widely taught and discussed.

19. Method of Doubt Descartes realizes that he has believed many things in the past he now knows to be false. How can he be sure that his current beliefs are not also false?

20. Method of Doubt In order to convince himself that he knows what he thinks he does, he wants a good foundation on which his beliefs can be based. “Reason now leads me to think that I should hold back my assent from opinions which are not completely certain and indubitable just as carefully as I do from those which are patently false.” (82)

21. Method of Doubt “So, for the purpose of rejecting all my opinions, it will be enough if I find in each of them at least some reason for doubt.”

22. Method of Doubt Instead of testing all of his beliefs individually, he instead proposes to examine the sources of his beliefs. We get all of our knowledge of the world by use of our senses. So Descartes starts there.

23. Method of Doubt Descartes’ proceeds methodically through three levels of doubt, gradually calling more of his knowledge of the world into question.

24. Levels of Doubt First Level (Illusion) Calls into doubt some things about the external world. Second Level (Dreaming/Matrix): Calls into doubt everything about the external world Third Level (Evil Demon): Calls into doubt the very existence of the external world.

25. First Level of Doubt • The senses sometimes deceive us. They are subject to illusions and misperceptions. • A square tower in the distance looks round • A straight stick in water looks bent • A rope in the dark looks like a snake • Mirages • Optical Illusions

28. First Level of Doubt • We ought not tofullytrust what has deceived us even once. • Therefore, we can doubt what our senses tell us. • Hence, we cannot know anything using our senses.

29. First Level of Doubt • Our senses mess up when things are far away, small, in poor viewing conditions and so on. • This does not show that our senses are generally untrustworthy.

30. First Level of Doubt “Unless perhaps I were to liken myself to madmen, whose brains are so damaged by the persistent vapors of melancholia that they firmly maintain that they are kings when they are paupers, or say they are dressed in purple when they are naked, or that their heads are made of earthenware, or that they are pumpkins, or made of glass. But such people are insane, and I would be thought equally mad if I took anything from them as a model for myself.” (116)

31. Second Level of Doubt But what about dreams? In realistic dreams we believe lots of things about our immediate environment in what seem to be optimal viewing conditions.

32. Second Level of Doubt • What if you are dreaming right now? Can you know that you aren’t? • What if you are always dreaming? Can you know that you aren’t?

33. Second Level of Doubt In the movie Inception the characters use tests (totems) to figure out whether or not they are currently dreaming. Descartes argues that any such test will be useless.

34. Second Level of Doubt Anything you could do in the real world, you could in principle do in a very realistic dream. Furthermore, the tests in Inception rely on the characters knowing at some previous time that they were awake.

35. A Modern Version: Brain in a Vat

36. For Next Time Read the G.E. Moore selections: (103-111)

37. Second Level of Doubt • Do you know right now that you are not in the Matrix? • If you don’t know this, how can you know that what you are seeing is actually there?

38. Second Level of Doubt Even if I am dreaming I can still know some things: • I have a body • Colors exist • Motion exists • Mathematical truths

39. Third Level of Doubt Imagine that: “…some malicious demon of the utmost power and cunning has employed all his energies to deceive me. I shall think that the sky, the air, the earth, colours, shapes, sounds, and all external things are merely the delusions of dreams which he has devised to ensnare my judgment.” (55)

40. Third Level of Doubt I can’t rule out this case either. Furthermore, in the evil demon scenario there is no external world at all! Now it seems that all of my beliefs are called into doubt.

41. Third Level of Doubt Are there any limits to what can be doubted? Descartes thinks there are!

42. Cogito “…if I convinced myself of something then I certainly existed. But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case too, I undoubtedly exist, if he is deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something…I must finally conclude that this proposition, I am, I exist is necessarily true whenever it is put forward by me or conceived in my mind.” (56)

43. Cogito Cogito ergo sum. I think, therefore I am.

44. Cogito Descartes thinks that the evil demon scenario cannot call into doubt any of our knowledge of our own mental states. But that still means we can doubt even the existence of an external world!

45. Formulating the Skeptical Argument All that we have established thus far is that the skeptical scenarios are possible. But why should this undermine our knowledge?

46. Certainty Argument The Certainty Argument • No one is ever absolutely certain of anything about the external world. • If S knows that p then S is absolutely certain that p. • Therefore, no one knows anything about the external world.

47. Possibility of Error Possibility of Error Argument • For any belief, p, S has about the external world S could be mistaken. • If a belief could be mistaken, then it is not a case of knowledge • Therefore, S does not know anything about the external world.

48. The Cartesian Theater There is a picture of how perception works that is underlying Descartes arguments. Russell makes it explicit.

49. The Cartesian Theater The idea is that what we immediately perceive are images/sensations and not physical objects. Russell calls the immediate objects of perception sense-data. (I’ll stick to sensations)

50. The Cartesian Theater