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 = perception
2. Knowledge = true belief
3. 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, nonphilosophers 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.