Knowledge,
Skepticism,
and Descartes
Knowing
In normal life, we distinguish between
knowing and just believing.
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“I think the keys are in my pocket.”
“I know the keys are in my pocket.”
“I believe 0.999999999… = 1.”
“I know 0.999999999… = 1.”
The traditional definition of
knowledge
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In cases where we know a claim, we have good
reason to think it is true: we feel the keys in our
hands, or we do the math, etc.
Knowledge is justified true belief. (This definition
goes back to Plato.) To know a claim, you must
believe it, it must be true, and you must have
good reason to believe it is true.
A skeptic is someone who argues that we can’t
justify certain types of claims.
Skepticism
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Skepticism = the view that we cannot know that
claims of some type are true (= we’re not able to
have good reasons to believe they are true).
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So a religious skeptic thinks we can’t know religious
claims, a moral skeptic thinks we can’t know moral
claims, etc.
We will examine two kinds of skepticism that interest
philosophers: skepticism about the external world and
skepticism about induction (reasoning about the
unobserved).
How could you be skeptical of the
external world?
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Normally, we think we know lots of things.
I know I am wearing shoes right now, I
know that Sacramento is the capital of
California, and so on. It would seem crazy
to say “I don’t know, I only believe” for any
of these beliefs.
But there are arguments that we can’t
know such claims.
Rene Descartes (1596-1650)
 Descartes
was (in our terms) a
scientist and mathematician.
 He invented analytic geometry,
demonstrating that geometry can
be reduced to algebra.
 Compare the ancient way of
doing geometry with Descartes’:
Euclid’s definition of a circle
‘A plane figure contained by one line such
that all straight lines falling upon it from
one point amongst those lying within the
circle (called the center) are equal to one
another.’
Descartes’ definition of a circle
‘A circle is all x and y satisfying x² + y² = r²
for some constant number r.’
Descartes’ mathematical and
scientific achievements
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Using analytic geometry, problems such as
finding the distance between any two points
becomes simple algebra.
He introduced the use of x, y, and z as variables,
and a, b, and c as constants, as well as the
standard notations for cubes and roots.
He used trigonometry to find the sine-law of the
refraction of light (Snell’s law).
And he applied this to explain rainbows.
Descartes vs. skepticism
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Descartes lived in a time of intellectual
turmoil and uncertainty (the Reformation
and the beginning of the Scientific
Revolution).
He rejected trust in authority and the
senses: they were often wrong.
But he also wanted to disprove skeptics, to
provide a foundation for science.
Descartes’ Meditations (1640)
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At the start of the First Meditation,
Descartes points out that, over the course
of our lives, we have many false beliefs.
Descartes wants to find an absolutely
certain start for knowledge: beliefs he can
use as the foundations of science.
These beliefs must be absolutely certain:
they must be immune to any doubt.
Cartesian Doubt
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So Descartes uses doubt, but only to find
absolute certainty.
Whatever beliefs can withstand the most
extreme doubt are absolutely certain.
Descartes imagines three extreme forms
of doubt:
1.
2.
3.
My senses are unreliable
I could be dreaming
I could be controlled by a godlike demon
Descartes’ illusion argument
1.
2.
3.
My senses sometimes deceive me in
ways that I can’t tell the difference
between true and false.
If (1), then any belief based on the
senses could be false.
So, I don’t know beliefs based on the
senses.
Descartes’ dream argument
1.
2.
3.
There is no criterion by which I can be
sure that I am not dreaming.
If (1), then every belief based on
experience could easily be false.
So, every belief based on experience is
not knowledge.
Descartes’ demon argument
1.
2.
3.
I cannot prove that
there is no demon.
If (1), then all of my
beliefs could easily
be false.
So, no beliefs are
knowledge.
The brain in a vat
Skepticism about the external world
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To sum up: to know something, we need
good reason to believe it is true.
But if I were a demon-prisoner or brain in a
vat, all my reasons would be exactly the
same, yet all my beliefs would be false.
So those reasons are not good reasons.
So there is no good reason to believe
there is an external world.
What the skeptic isn’t claiming
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Skeptics don’t claim to know we are brains in a
vat. They are saying no one knows one way or
the other: we have no good reasons to show that
we are or aren’t envatted.
Skeptics aren’t just saying we can’t be certain
the world is real. They are saying more than
that: they are saying you have no good reason
to believe the external world over the virtual
world in a vat.
Skeptics aren’t saying we know nothing. (That
would be knowledge!) They are saying we don’t
know there is an external world.
Descartes’ solution
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So, after the First Meditation, Descartes’
goal is clear: he must defeat the demon
hypothesis. That is, he needs to prove
that there is no demon. If he can’t, then
he doesn’t have knowledge of the world.
The flip side, though, is optimistic: if he
can defeat the demon, then he has a
foundation for knowledge immune to the
most extreme skepticism.
Descartes’ solution:
Cogito ergo sum
1.
2.
3.
I am thinking.
Whatever thinks, exists.
Therefore, I exist.
or:
1.
2.
3.
I could be dreaming or be deceived by a
demon.
But even if I am dreaming, or some demon is
deceiving me, then I exist.
Therefore, I exist.
Descartes’ way back to knowledge
Given that “I exist” is absolutely certain, how
can we get to the sort of scientific
knowledge that Descartes wants to
justify?
Descartes proposes two reinforcing ways:
1.
2.
Prove God exists.
Take clear and distinct ideas as the
standard of certain knowledge.
Descartes’ proof of God
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We have an idea of God as an infinite,
real being.
The cause of any idea must have at least
as much reality as the idea has.
So, there is something with infinite, real
being.
If there is something with infinite, real
being, then it is God.
So, God exists.
God and knowledge
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If God exists and is
perfect, then God
wouldn’t allow us to
be deceived by a
demon.
So Descartes thinks
he has disproved the
demon hypothesis.
Clear and distinct ideas
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Additionally, Descartes claims that clear and
distinct ideas are true.
Knowing that ‘I exist’ is an idea I cannot doubt.
Anything that meets that standard (that is clear
and distinct) is absolutely certain.
This is not sense perception, but pure reason:
mathematical proof is the best example.
So reason gives us genuine knowledge. This is
what makes Descartes a rationalist.
Descartes: Summary
Descartes’ anti-skeptical, rationalist theory
of knowledge:
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“Cogito ergo sum” provides a first example of
something known, and reveals what is
needed: clear and distinct ideas.
Then we prove clearly and distinctly that the
idea of God implies a perfect cause, i.e., God.
A perfect God cannot deceive, so our faculties
must be reliable if used properly (if guided by
reason, e.g., math).
Parting thoughts on Descartes
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Descartes thought he had defeated
skepticism and laid a rationalist foundation
for knowledge (mathematical science).
But his extreme version of skepticism was
not as easy to defeat as he thought.
Reason was key to science, but it still
wasn’t clear how we could prove that we
have knowledge.
The Regress of Justification
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Suppose that I believe P, and P is to be justified.
Its justification will be other beliefs. But then if P is
to be justified, these other beliefs must be justified
too, and so on…
How to prevent an infinite regress? Perhaps some
beliefs are justified in a way that does not depend
on any other belief. Descartes took this route:
foundationalism, taking some beliefs to be selfevident or self-justifying.
Avoiding skepticism by
redefining knowledge
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But perhaps the problem was assuming
that to know = to have justified true belief.
The JTB definition has serious problems: if
knowledge must be justified, then an
infinite regress implies there is no
knowledge. And Gettier counter-examples
show JTB is too broad.
So what’s important in “knowing”?
The Causal Theory of
Knowledge
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An internalist claims knowledge must be
justifiable in reasons that the believer is
able to be conscious of.
An externalist claims that knowledge
depends on having a reliable causal
connection to whatever makes a belief
true, even if that connection is unknown to
the believer (“outside the believer’s mind”).
Externalism
(The Causal Theory of Knowledge)
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Externalists replace justification (giving
reasons) with reliable causation (what
happens to make you believe it).
The “reliability” in question is that it reliably
causes true beliefs.
The believer doesn’t have to be aware of
how knowing causes true beliefs.
Example: you may recognize a musical
key without being able to explain it.
A challenge for the externalist
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While externalism promisingly solves
regress and Gettier problems, it raises a
puzzle of its own: what if a person doubts
their own reliably caused beliefs?
Suppose a doctor has a real causal ability
to recognize illness. But she dismisses it
because she doesn’t want to risk a
patient’s life on her hunch. Isn’t knowing
supposed to guide action?
What good is skepticism?
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Skeptics don’t advance beliefs, since they
don’t claim to have good reasons.
Someone without beliefs is less likely to do
terrible things, and more likely to consider
counter-evidence and change their minds.
(82% of philosophers are realists about the
external world, 5% skeptics, 4% idealists, 9%
“other.”)