Skepticism

Summary of Last Time

Last time we were concerned with the question: “What is knowledge?”

We wanted to know what is the essence of knowledge, what it is that makes something knowledge, what is the correct definition of ‘knowledge’.

Three Theories

We discussed three theories considered by Socrates and Theaetetus in Plato’s dialogue Theaetetus:

1. Knowledge = perception2. Knowledge = true belief3. Knowledge = justified true belief

Knowledge = Perception

The main argument that Plato/ Socrates presents against the claim that knowledge = perception is this:There are lots of things that can be known, but which cannot be perceived, like facts about essences, facts about numbers, facts about identity and distinctness. Therefore knowledge cannot be identical to perception.

Knowledge = True Belief

Main argument against knowledge = true belief:People often have beliefs based on bad reasons, reasons that don’t support their beliefs at all. So for instance, you might believe that you were going to get in a car accident because a fortune teller told you that you were. Sometimes these beliefs are true “by accident”– but, intuitively, beliefs that are true “by accident” are not knowledge.

Knowledge = Justified True Belief

Finally, we considered the most popular account of knowledge for most of the history of Western philosophy: knowledge = justified true belief, a belief that is true, and is believed for good reasons (unlike the fortune teller example).

The main objection to this account is similar to the objection to knowledge = true belief.

Accidental Truths

Even beliefs that you hold for good reasons, and which are true, can still be “accidentally” true, in the sense that the reason why they are true is totally unrelated to the reasons you have for believing them.

Russell’s Stopped Clock

In Russell’s stopped clock case, the fact that the clock says “8:00” is a good reason for believing that it is 8:00AM. And it may be true that it is 8:00AM. But the fact that it is 8:00AM has nothing to do with what the clock says: the clock has stopped, it will say “8:00” no matter what time it is. Even though your reason is a good one, it is only accidental that your belief is true. Intuitively, you don’t know that it’s 8:00AM.

Knowledge First

Finally, I introduced the idea that maybe there is no definition of ‘knowledge’ so the project of trying to find one is doomed to failure.

This is the “knowledge first” view made popular by Timothy Williamson at Oxford.

Skepticism

Today we’re going to set aside the question of what knowledge is and we are going to ask:

Is it possible for us to know things?

A surprisingly large number of philosophers have answered “NO”– that knowledge is impossible.

ANCIENT SKEPTICISM

Sextus Empiricus (160-210 C.E.)

Sextus Empiricus

Sextus Empiricus was a Roman philosopher and physician in the 2nd and 3rd Centuries. The “Empiricus” in his name is from the fact that he belonged to the Empiric school of medicine, which said that medicine should be based on our experiences of what works, and that we should not seek after the hidden, unobservable causes of illness.

Sextus Empiricus

In philosophy, Sextus was a follower of a Greek school of thought named Pyrrhonism, after its founder Pyrrho, about whom little is known.

Our most complete account of the ideas and arguments of Greek and Roman skepticism comes to us through the writings of Sextus.

Three Views

Sextus distinguishes between three views on the question of whether it is possible to know anything:1. Dogmatism: knowledge is possible.2. Academic Skepticism: knowledge is not

possible.3. Pyrrhonian Skepticism: I don’t know whether

knowledge is possible or not.

Appearances vs. Judgments

Sextus makes a distinction between appearances (perceptions, how things look or taste or feel) and judgments (beliefs, how we think things are). The skeptic does not deny appearances– when he feels cold, he will accept that he feels cold– but he refuses to accept any judgment about how things are– that it is cold.

Equipollence

The main reason Sextus has for denying Dogmatism and accepting Skepticism is the “equipollence” or “equal force” of arguments on both sides of any issue (except appearances).

For any philosophical, logical, mathematical, or scientific claim, you can give equally convincing arguments for and against it.

Observer Effects

Sometimes appearances differ from person to person, or between people and animals. For example, a certain temperature might feel cold to you, and fine to me, and hot to a polar bear. How can we say what the world outside of appearances is like, when appearances vary from observer to observer in this way.

Effects of Object’s Condition

Sometimes too, the appearances vary, even if they are the same to all observers. For example, snow is white, but snow is water, and water is blue. Or almonds are sweet, but a ground almond is not sweet, but oily. So how can we say how things really are outside of appearances, when they present us with contradictory appearances in different circumstances?

The Historical Argument

Another skeptical argument Sextus outlines goes like this:

Suppose someone believes a philosophical or scientific view X. Before the view was invented, nobody believed it, they believed something else, Y. So how do you know that some other new view won’t come along and displace X?

The Regress Argument

Suppose you have some criterion C for what is true, a way of finding out the truth. If something satisfies the criterion, then it is true, and if something does not satisfy the criterion, it is not true.

The Regress Argument

But how can you know that C is such a criterion?

You must have some way to tell whether C is a criterion for truth. That is, you must have a criterion C* that tells you “it is true that C is a criterion for truth,” because it has C*.

The Regress Argument

C* can’t be identical to C, because it is circular to justify a criterion with itself.

(Parody: “How do I know that every sentence less than 20 words long is true? Because the sentence “every sentence less than 20 words is true” is less than 20 words, and therefore true!”)

The Regress Argument

Therefore, C* has to be different from C. But how do you know that C* is a criterion for truth? By the same reasoning, there must be some other criterion C** whereby you can tell that “C* is a criterion for truth” is true because it has C**. And there must be a C*** and a C****, and so on, infinitely. But this is impossible.

Two Kinds of Skeptical Argument

The first two arguments we considered were “equipollence” arguments. How things look or feel differs “equally” in a way that we can’t say for sure how they objectively are.The historical and regress arguments are noticeably different: you can deny that the evidence on both sides is “equal,” yet still affirm that there’s no reason to believe anything.

The Academy

Academic Skepticism

Plato founded the first Western university, the Academy, in Athens. After he died, there was a movement at the Academy toward skepticism.Sextus characterizes Academic Skepticism as the view that nothing is knowable. This was objectionable, because it is insufficiently skeptical: anyone who believes that believes something, but the equipollence of reasons tells you to believe nothing.

Academic Skepticism

From what we know, Sextus is probably wrong about what the Academic Skeptics believed: Carneades, the most important of them, thought that you couldn’t know that you couldn’t know anything.

Still, the point is a good one: there’s something weird about believing you know that nothing is knowable.

Academic Skepticism

The Academics were different from the Pyrrhonian skeptics, however.

For example, Carneades held the doctrine of probability: that some things were more probable than others, and you should behave as if the most probable ones were true. This is a denial of equipollence.

Therapeutic Philosophy

Last time we learned that Protagoras believed that there was no absolute truth, there was only true-for-you and true-for-me. Therefore, the point of philosophy cannot be to discover the truth or to convince people of the truth– they already have what is true-for-them. Protagoras thought that the goal of philosophy was therapeutic: making people’s lives better.

Skepticism as Therapeutic

Skeptics aren’t truth-relativists (they don’t believe truth is relative or that it is absolute, because they don’t believe any philosophical claim). But they cannot hold that the goal of philosophy is to discover the truth, because they don’t believe (or disbelieve) that this is possible. Sextus also says that the goal of skepticism is to improve the lives of the people who believe it.

Ataraxia

Sextus tells the story of the famous Greek painter Apelles. According to the story, Apelles was trying to paint a horse’s “foamy saliva,” but kept failing. Angry at his failure, he threw a sponge at the painting and it made a perfect image of foamy saliva. Suddenly, Apelles felt “ataraxia,” an inner quiet and freedom from worry and distress.

Ataraxia

What Apelles wanted when he tried, he only got when he had given up. The Pyrrhonians believed that skepticism led to ataraxia. It keeps you from contradictions (remember that equipollence says everything has equal arguments in favor of it), it keeps you from desire (because you don’t believe anything is good), it keeps you from trying to change unavoidable things (because you don’t know how to change them).

EARLY MODERN SKEPTICISM

Extremely Brief History of the West

The history of Western civilization may be divided roughly into the following periods:Classical Antiquity: When Greek and Roman power was at its height.Middle Ages: Catholic dominance.Renaissance: A “rebirth” of art and literature.Modern Period: Science reaches its modern form, discovery of Newton’s Laws, chemistry…

Science Brings Skepticism

The Modern Period was a period of scientific change and revolution.

The new theories, however, encouraged skepticism, and many early Modern philosophers were faced with the task of deciding what to do about skeptical arguments.

The Historical Argument

As new scientific theories replaced old ones, the historical argument for skepticism became more prominent. For example, for the longest time people had believed Galen’s view that the liver and heart both created blood that was then consumed by the rest of the body, until Harvey showed that the heart was a pump and circulated blood throughout the body.

The Historical Argument

If what we believed for so long was wrong, people worried, how do we know that Harvey’s theory too will stand the test of time? Perhaps a new theory will come along and unseat it.

This was a common Pyrrhic argument, but it was a lot more powerful when revolution was happening all over science.

The Copernican Threat

In addition to providing yet another context for the historical argument, Nicolaus Copernicus’ (1473-1543) heliocentrism– the idea that the Earth went around the sun– provoked skepticism in another way. If something as obvious to us as “the earth does not move” is false, then why should we trust other seemingly obvious truths?

The Threat from Atomism

Another new scientific theory (one also held by certain Greeks) was the view called atomism: that there were tiny things, atoms, out of which everything else was made. The atoms had shapes, sizes, and they moved around, but they did not have colors, or sounds, or temperature. Color, sound, taste, heat, etc. was all just the effects of atoms moving against our sensory organs.

The Threat from Atomism

This certainly suggested skepticism! According to science there were no colors in the world, just in our minds. There was no sweetness, nothing was loud or soft, fire wasn’t hot… everything was atoms and the void. If we are denying that blood is red and snow is white, couldn’t science convince us that anything was false. And if it could, how do we know now what is true?

Michel de Montaigne

Michel de Montaigne

Some philosophers became Pyrrhics who rejected the possibility of knowledge (without accepting its impossibility).

Michel de Montaigne, perhaps the most famous essayist of the Renaissance, was an advocate of skepticism.

Natural Theology

Montaigne had translated a work on Natural Theology by Raymond Sebond. Natural Theology is the idea that you can use reason and evidence to arrive at religious truths, not just the bible. It was of course not a view that the Catholic church liked very much, because it basically questioned the church’s authority to decide the truth.

Montaigne’s Apology

The work was put on the list of “banned books” by the church, and so Montaigne wrote an impassioned defense of it “An Apology of Raymond Sebond.” (“Apology” in this context meant “defense,” just as Plato’s “Apology” for Socrates was Socrates’ defense of his views.) But instead of actually defending Sebond’s views, Montaigne defended the idea that since we can’t know anything, we can’t know that Sebond was wrong!

Skeptical Arguments

Mostly, Montaigne just made the same arguments as the ancient skeptics. Here was his version of the infinite regress: Q: Why should we think that fire is hot?A: Because it feels hot.Q: Why should we think that if something feels hot, then it is hot?A: Because when something feels a certain way, it is that way.

Infinite Regress

Q: Why should we think that IF it’s true that if something feels a certain way, it is that way THEN it’s true that if something feels hot, then it is hot?…

René Descartes (1596-1650)

Descartes

René Descartes was ‘the father of Modern philosophy,’ the first to make a strong break with the earlier Medieval tradition, that was still very oriented toward ancient Greek and Roman thought.

We’ve already met Descartes in this class as a defender of substance dualism, the idea that the mind and the body are two different substances.

Descartes’ Project

Descartes’ substance dualism was part of a larger project, whereby Descartes thought he could reform all of philosophy, make it compatible with the emerging new science, and establish the principal truths of religion– the existence of God and the immortality of the soul– through reason without recourse to scripture.

Cartesian Skepticism

The main engine for this project was skepticism. Descartes’ idea was that we should doubt everything that it is possible to doubt. Then we should look for what is left over: what is it not possible to doubt? Anything that it is not possible to doubt will be absolutely certain. We can then reason from only the absolute certainties to other absolute certainties, so that there will be no error in any of our beliefs.

The Meditations

Descartes’ greatest philosophical work was “Meditations on First Philosophy,” often just called the Meditations. The First Meditation presents a series of skeptical scenarios that put Descartes deeper and deeper into doubt. By the end it almost seems as if there’s nothing left for him to be certain of.

Knowing P

Knowledge seems to work like this: in order to know that p (where p is some proposition), my evidence must rule out the truth of not-p.For example, suppose I want to go to the bank and it’s a Saturday. Some banks are closed on Saturday. Unless I have evidence that rules out the possibility that my bank is closed (like I call and they say it’s open) I don’t know it’s open.

Second Example

Or suppose that I am a strict vegetarian and I am ordering food at a restaurant. I could order the green beans, but sometimes green beans have pork in them. Do I know that these green beans don’t have pork in them? Intuitively not. In order to know, I have to get evidence that rules out the pork-possibility. For example, I could ask the waiter and he could tell me.

Another way to state the principle is this:

Suppose I have some evidence E and on the basis of E I believe that p. Do I know p?

If I can imagine that it is true that I have E and p is nevertheless false, then I don’t know that p.

Strategy: Meditation I

Descartes’ strategy in the First Meditation is to apply this principle to different categories of things that can be known.

Far Away and Tiny Things

For example, there are things that are very tiny or very far away. Sometimes horses that are very far away look like cows. If I see something that is very far away that looks like a cow, I can’t be sure that it is not a horse. I could have the same evidence (how it looks) and yet be wrong. So I don’t know that it’s a cow. Thus we must doubt beliefs formed by using the unaided eye to look at very far away or very tiny things.

Minor Skepticism

This of course is not a very serious skepticism. There are some things we have to doubt, but we can always change that by using microscopes or telescopes, or by walking closer to far away things. But Descartes is just getting started…

Dream Skepticism

What about middle-sized object that are close up. Can’t I know that I have hands? Or that I’m teaching class? Descartes says no.Sometimes I dream. The dream seems to me as if it is real. I might dream that I was teaching class, even when I was at home sleeping in my bed. Since I would have the same evidence I now have, but I would not be teaching class, then I cannot know that right now I’m teaching.

Dream Skepticism

The same is also true for other beliefs about ordinary middle-sized objects. I might lose my hands in a horrible accident, but then have a dream in which I still had hands. So even the fact that I’m looking down and it looks like I still have hands is not enough to allow me to conclude that I do in fact have hands.

Math and Geometry

But, says Descartes, it seems as though there are still some things that I could know even if I’m dreaming.Dreams are a lot like ‘moving paintings.’ And there are certain things that are impossible to paint. I can’t paint a picture where there are two apples at the top, two apples at the bottom, but only three apples total in the picture.

Math and Geometry

Similarly, Descartes thinks, you can’t have a dream where in your dream you are looking at a triangle-shaped slice of pizza, but this is an unusual triangle: this triangle has 18 sides. That’s impossible, even in paintings and dreams, all triangles have 3 sides. So even if I’m dreaming right now, I can know mathematical truths like 2 + 2 = 4 and geometrical truths like all triangles have 3 sides.

God’s Omnipotence

But, Descartes says, I can’t know math and geometry either. God is all powerful. Maybe God exists and maybe he doesn’t– at this point Descartes can’t rule out the possibility that God exists. Since God is all powerful, he could make me think that triangles had 3 sides, even if in reality they didn’t.

Evil Demon

Of course, God is also all good. So he would never trick me into thinking false things.

However, I cannot rule out the possibility that there is a being that is like God in terms of power, but who is evil and committed to deceiving me. Let’s call this being an Evil Demon.

The Evil Demon Argument

1. My evidence does not rule out the existence of an Evil Demon, who is all powerful, and whose sole aim is to deceive me.

2. Therefore, my evidence does not rule out the possibility that I could think 2 + 2 = 4, but still be wrong, because I was deceived by the all powerful Evil Demon.

3. Therefore, I do not know that 2 + 2 = 4.

Brain in a Vat

A more contemporary way to frame this problem is the brain in a vat hypothesis:Suppose scientists kidnapped you last night, removed your brain from your body, and put it in a vat of nutrients. All your current experiences are being fed to your brain through electrodes, and are controlled by powerful super computers, simulating a complex reality.

Brain in a Vat Argument

1. You don’t know that you’re not a brain in a vat.

2. If you’re a brain in a vat, then you don’t have hands, and maybe even 2 + 2 = 17.

3. Therefore you don’t know that you have hands, or that 2 + 2 = 4, or almost anything else.

The Cogito

Does that mean that nothing is knowable? Descartes says wait one moment:

Right now I am thinking that I might be deceived by an all powerful Evil Demon. That could be true, it could be false, I don’t know. But the fact that I’m thinking something shows that I exist. I think, therefore I am.

Knowledge of Appearances

My evidence does rule out the possibility that I do not exist. If I didn’t exist, I wouldn’t have any evidence– any experiences, or thoughts, or impressions– but I do have those things.In fact, I know a lot more than that I exist: I know that I’m thinking, that I’m doubting, and I know that I seem to have hands (even if I don’t) and it seems like 2 + 2 = 4 (even if it doesn’t).

The “Veil of Perception”

Descartes concludes that he can be certain that he exists and that the world seems to be a certain way to him– that he has the appearances he does– even if the world isn’t actually that way, even if reality does not match the appearances. This is the “veil of perception”: the world of appearances. The problem then becomes how do I know anything about the world outside my appearances?

The Veil of Perception

Perhaps one way to see the problem is this: suppose I gave you a painting of a person you had never seen before. I ask you “Does this look like the person?” You can’t answer, you don’t know. But this is exactly how your experience is: you’ve only ever experienced how you perceive the world, never how it is by itself. Is how you perceive it like it really is? You can’t answer, you don’t know.

Unfortunately, nobody likes Descartes’ solution to the problem. It’s too complicated for our purposes, but suffice to say that it convinced no one. Most people thought that Descartes “cheated” by adding in a bunch of extra assumptions that were not certain, boldly asserting they were certain– without giving any argument for them.

The Skeptical Problem

However, Descartes’ big contribution was to put the skeptical problem at the forefront. How do we look beyond the veil of perception? How do we know anything other than how things appear to us? Couldn’t reality be wildly different from how it appears to us? Couldn’t there simply be no reality beyond the appearances?

Indirect Realism

The picture that philosophy inherited from Descartes was called “indirect realism.” There were three things involved in your experience: You, an idea (appearance), and the object that the idea represented. Since you only directly perceived ideas, the skeptical challenge was to explain how you knew that the objects of those ideas existed.

George Berkeley (1685-1753)

George Berkeley

George Berkeley was an Irish philosopher and Bishop in the Church of Ireland.

He advocated an unusual response to skepticism, which we now call “subjective idealism.”

Subjective Idealism

Berkeley denied atomism and materialism, the idea that things were made out of atoms and matter.

Instead, Berkeley thought that appearance was identical to reality, and that things like tables, and mountains, and human bodies, were made out of ideas.

Esse est Percipi

Berkeley’s slogan was “esse est percipi,” to be is to be perceived. Seeming to exist and existing were the same thing; seeming to be red and being red were the same thing. How could we know that reality was a certain way when our appearances were a certain way? Because appearance = reality.

Note

Be careful, there are two English words:

‘idealism’ = ‘ideal’ + ‘ism’ = the doctrine that you should be guided by lofty goals and not practical concerns.

‘idealism’ = ‘idea’ + ‘lism’ = the doctrine that to be is to be perceived.

Objects of Perception are Ideas

Berkeley had a bunch of arguments for this view. Here are a few examples:1. You can see colors.2. Colors are ideas, because as the atomists

admit, atoms have no colors.3. So what you see are ideas.4. You can see tables.5. Therefore, tables are ideas.

Resemblance and Representation

1. A painting represents something by resembling it. A painting is a painting of you because it looks like you.

2. Ideas are like paintings in the mind: they represent reality by resembling it.

3. Ideas are mental. Matter and atoms don’t at all resemble them.

4. Therefore, any idea must represent an idea. If you think of a tree, you are thinking of an idea

Samuel Johnson (1709-1784)

Samuel Johnson was a poet and essayist in Britain, one of the most important British literary figures, and a contemporary of Berkeley.

James Boswell, his biographer, conveys the following story in Life of Samuel Johnson.

Samuel Johnson vs. Berkeley

“After we came out of the church, we stood talking for some time together of Bishop Berkeley's ingenious sophistry to prove the nonexistence of matter, and that every thing in the universe is merely ideal. I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it – “I refute it thus.””

Idealism

Idealism in various forms was the mainstream philosophical view in the West from the late 18th Century right up to the early 20th Century, though it was popularized more by Hume, Kant, and Hegel than by Berkeley. However, nowadays it is fairly unfashionable in Anglophone philosophy, while still having a fair number of adherents in Europe.

CONTEMPORARY RESPONSES TO SKEPTICISM

G.E. Moore (1873-1958)

G.E. Moore

George Edward Moore, most commonly known as G.E. Moore, was Professor of Philosophy at Cambridge, and is considered one of the founders of “analytic philosophy,” the dominant philosophical tradition in the Anglophone world.

British Idealism

Before Moore, the tradition in British philosophy was “British Idealism” which was influenced principally by Hegel (the most prominent German philosopher of the 1800’s). British Idealism was a version of Absolute Idealism, which combines two claims: (1) all facts are mental facts and (2) there is really only one fact, the Absolute, a unified reality with no separable parts.

Moore and Russell

Moore, along with another Cambridge philosopher, Bertrand Russell, were Pluralists and Realists. They were “Pluralists” in the sense that they thought there were many things instead of one Absolute, and they were Realists in the sense that they thought there was a mind-independent reality. This, as I said, is the dominant Anglophone philosophical position today.

Inconsistent Claims

Consider these three claims:

E: Every philosopher is smart.P: Michael is a philosopher.S: Michael is not smart.

Taken together, E, P, and S are inconsistent: they cannot all be true at once.

Reject S

What can you do if you find yourself believing contradictory claims? Well, you have to give one up. For example, here’s a valid argument:

E: Every philosopher is smart.P: Michael is a philosopher._________Therefore, S is false: Michael is smart.

Reject P

Here is another valid argument:

E: Every philosopher is smart.S: Michael is not smart.____________Therefore, P is false: Michael is not a philosopher.

Reject E

And this argument too is valid:

P: Michael is a philosopher.S: Michael is not smart.________________Therefore, E is false: it is not true that every philosopher is smart.

Inconsistency and Validity

Whenever you have a set of inconsistent claims A, B, and C, then the arguments:

1. A and B, therefore not C.2. A and C, therefore not B.3. B and C, therefore not A.

Are all valid arguments.

The Skeptical Problem

Moore analyzes the skeptical problem as follows: we are inclined to accept the following set of inconsistent claims:

H: I know that I have hands.B: I don’t know that I’m not a brain in a vat.I: If I don’t know that I’m not a brain in a vat, then I don’t know that I have hands.

The Skeptical Argument

The skeptic then argues to the conclusion that we don’t know we have hands:

B: I don’t know that I’m not a brain in a vat.I: If I don’t know that I’m not a brain in a vat, then I don’t know that I have hands._______Therefore, H is false: I don’t know that I have hands.

Moore’s Alternative

But is this the best route? We could also argue as follows:

H: I know that I have hands.I: If I don’t know that I’m not a brain in a vat, then I don’t know that I have hands.________Therefore, B is false: I do know that I’m not a brain in a vat.

When we are faced with the inconsistent set of beliefs, we can reject our ordinary claims to knowledge (“I have hands”) or we can reject the skeptical scenario (“I might be a brain in a vat”).

Moore says we should always reject the skeptical scenario. When you have inconsistent beliefs, reject the belief that is least certain.

It’s extremely certain, Moore argues, that I have hands, and that I know that I have hands. Look, they’re right there!I’m less certain that I might be a brain in a vat. Sure it sounds reasonable that I might be a brain in a vat, for all I know, but I must give up one of these beliefs, and I’m not giving up “I have hands.”

Moorean Facts

Facts like “I have a hand” are often called “Moorean facts”: skeptical hypotheses (Dream Argument, Evil Demon, brain in a vat) can never cause us to doubt them, because they are always more certain than the skeptical hypotheses.

SUMMARY

Ancient Skepticism

Skepticism has a long tradition in Western philosophy dating back to Classical Antiquity (the ancient Greeks and Romans).

Ancient skepticism was often a lifestyle choice: people thought they could achieve a peaceful inner life by suspending judgment on everything.

Modern Skepticism

Skepticism paradoxically gained in prominence during one of the greatest periods of scientific discovery in the West.

New knowledge and scientific theories threatened our long held beliefs: if what we thought we knew for so long was false, what else might be false? Maybe everything.

Cartesian Skepticism

Descartes turned skepticism on its head: he thought that by doubting everything it was possible to doubt, he could arrive at absolute certainty in what remained, and then use those absolutely certain truths to establish everything else on an unshakeable foundation of certainty. Most philosophers think Descartes failed, but he did leave us with a clear statement of the skeptical problem.

Idealism

One of the most enduring responses to skepticism has been Idealism, the idea that we can know about reality, because we can know about appearances, and appearances = reality.Although this view has been popular, it flies in the face of common sense. Most ordinary, non-philosophers believe in a reality independent of how they perceive things.

Common Sense Defended

G.E. Moore reacted to the idealists of his time, and in doing so helped usher in a new tradition in British and American philosophy. He argued that skepticism forced us to choose between common sense and “skeptical hypotheses” like the claim that we didn’t know we weren’t brains in vats. He said we should always choose common sense, because it was always the most probable choice.