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 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 ( ) 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.
If our senses can be shown to be trustworthy than in one go he will have secured most of our knowledge of the world. If they fail, he will call into doubt all our knowledge of the world.

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 to fully trust 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.
Actually, Inception is pretty good (though maybe not entirely consistent) on this point. The totems are only used to tell whether or not you are in someone else’s dream. Not your own.

35. A Modern Version: Brain in a Vat

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

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.
After reading Descartes it is natural to think that something like this is right.

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

51. The Cartesian Theater
In the Matrix, the computer provides us with deceptive sensations.
But we can’t be wrong about what sensations we are experiencing!
In other words, even when deceived about the world, we aren’t deceived about how things appear to us.

52. The Cartesian Theater
Russell thinks that we must infer on the basis of our senses that:
An external physical world exists
That it is largely how we believe it to be

53. Not Deductive… Russell grants that the inference cannot be deductive:
It is possible to have things appear to us perceptually in the way they do and be in the Matrix
Ditto dreaming
Ditto Evil Demon

54. Russell’s Argument
Russell argues that the best explanation of our sensations is that they are not merely sense-data but are caused by external objects.

55. Russell’s Argument Reason #1: Persistence of Unobserved Objects
If all there are are sensations, then what happens when they are unobserved?
Do they cease to exist?
How do their properties change (e.g. cat gets hungry, moves around)

56. Russell’s Argument Reason #2: Other People
It is almost impossible to imagine the speech and actions of other people as not originating from a mind.
The best explanation for the behavior of other human-shaped objects is that they have minds like me.

57. Russell’s Argument Reason #3: Objects from Different Perspectives
A table will look different from different angles and in different light.
But it is still perceived as the same table.
The visual differences caused by perspective and illumination are highly predictable and regular

58. Russell’s Argument Reason #4: Instinctive Belief
We typically don’t reason to, or argue for, the claim that external objects exist.
We believe it instinctively without any prompting
We would never have called it into doubt without some obscure philosophical reasoning.
Instinctive beliefs like this should only be revised if they are shown to clash with other equally instinctive beliefs.

59. Russell’s Argument
Unfortunately this argument has serious problems.

60. Problem #1
The Matrix scenario seems to equally well explain all the systematic elements of our experiences that Russell mentions:
Persistence of objects
Other people’s behavior
Perspectival differences

61. Problem #1
If super-computers or an evil demon constructed a virtual world to try to trick you, one would expect that world to have all the observable regularities that a physical world would.

62. Problem #2
Russell claims that an external world is the simplest explanation.
This just seems patently false.
The physical world (if it exists) is immeasurably vast and so deeply complex that we have spent thousands of years trying to understand it.
The evil demon scenario seems very simple by contrast.

63. Problem #3
Perhaps Russell means that an external world is the best explanation of our experiences.
Maybe…
But you need to spell out what makes one explanation better than another.
It turns out to be very hard to do.
Even if we can do this, we may still be able to imagine a skeptical scenario that is at least as good an explanation by whatever measure we settle on.