External World

Skepticism

For Next Time

Read the Feldman selection (94-99)

IntroductionIntuitively we think we know lots of things:

Knowledge of our immediate environmentKnowledge 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 statesKnowledge of the mental states of others

IntroductionIntuitively we think we know lots of things:

Scientific knowledgeMoral knowledgeReligious knowledgeMathematical knowledge

Skepticism

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

IntroductionSkeptics 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.

Introduction

There are very few global skeptics.

Pyrrho360-270 BC

Introduction

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

IntroductionDomain specific skepticism can be far more defensible and interesting.

Skepticism about knowledge of the futureSkepticism about other mindsSkepticism about moral knowledgeSkepticism about scientific knowledgeSkepticism about religious knowledgeSkepticism about the external world

External World Skepticism

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

External World SkepticismMental States

ThoughtsBeliefsDesiresCurrent experiences and sensationsEmotions

External World SkepticismThings in the External World (everything else):

PlantsAnimalsOther peoplePlanets/galaxies etc.

External World SkepticismIf 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 environmentKnowledge about the pastKnowledge about the futureKnowledge of general facts about the worldKnowledge about the mental state of othersScientific KnowledgeMoral knowledge (?)Religious knowledge (?)

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

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.

Method of DoubtDescartes 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?

Method of DoubtIn 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)

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.”

Method of DoubtInstead 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.

Method of Doubt

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

Levels of DoubtFirst 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.

First Level of DoubtThe senses sometimes deceive us. They are subject to illusions and misperceptions.

A square tower in the distance looks roundA straight stick in water looks bentA rope in the dark looks like a snakeMiragesOptical Illusions

First Level of DoubtWe 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.

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.

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)

Second Level of DoubtBut what about dreams?

In realistic dreams we believe lots of things about our immediate environment in what seem to be optimal viewing conditions.

Second Level of DoubtWhat 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?

Second Level of DoubtIn 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.

Second Level of DoubtAnything 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.

A Modern Version: Brain in a Vat

For Next Time

Read the G.E. Moore selections: (103-111)

Second Level of DoubtDo 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?

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

Third Level of DoubtImagine 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)

Third Level of DoubtI 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.

Third Level of DoubtAre there any limits to what can be doubted?

Descartes thinks there are!

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)

Cogito

Cogito ergo sum.

I think, therefore I am.

CogitoDescartes 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!

Formulating the Skeptical ArgumentAll that we have established thus far is that the skeptical scenarios are possible.

But why should this undermine our knowledge?

Certainty ArgumentThe Certainty Argument

(1) No one is ever absolutely certain of anything about the external world.

(2) If S knows that p then S is absolutely certain that p.

(3) Therefore, no one knows anything about the external world.

Possibility of ErrorPossibility of Error Argument

(1) For any belief, p, S has about the external world S could be mistaken.

(2) If a belief could be mistaken, then it is not a case of knowledge

(3) Therefore, S does not know anything about the external world.

The Cartesian TheaterThere is a picture of how perception works that is underlying Descartes arguments.

Russell makes it explicit.

The Cartesian TheaterThe 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)

The Cartesian Theater

The Cartesian TheaterIn 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.

The Cartesian TheaterRussell 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

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

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.

Russell’s ArgumentReason #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)

Russell’s ArgumentReason #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.

Russell’s ArgumentReason #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

Russell’s ArgumentReason #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.

Russell’s Argument

Unfortunately this argument has serious problems.

Problem #1The 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

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.

Problem #2Russell 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.

Problem #3Perhaps 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.