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According to philosophical skepticism, we
can’t have knowledge of the external world.
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Knowledge by acquaintance (knowing what)—
experiential knowledge.
Performative knowledge (knowing how)—skill
knowledge.
Propositional knowledge (knowing that)—
factual knowledge.
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Rationalism is the doctrine that reason is the
only source of knowledge of the external
world.
Empiricism is the doctrine that sense
experience is the only source of knowledge of
the external world.
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An a-priori proposition is one that can be
known prior to or independently of sense
experience.
An a-posteriori proposition is one that can
only be known after one has acquired sense
experience.
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An analytic proposition is a logical truth or
one that can be turned into a logical truth by
substituting synonyms for synonyms.
A synthetic proposition is a proposition that
is not analytic.
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According to the correspondence theory, a
proposition is made true by its
correspondence to reality.
According to the coherence theory, a
proposition is made true by its coherence
with a system of beliefs.
According to the pragmatic theory, a
proposition is made true by its practical
consequences.
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According to cognitive subjectivism, a
proposition is made true by ones
believing it to be true.
According to cognitive cultural relativism,
a proposition is made true by a society
believing it to be true.
Objection: Both views are selfcontradictory because a proposition and
its negation could be believed to be true.
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The doctrine that a proposition is true if and
only if things are as it says they are.
This preserves the insight that true
propositions accurately represent reality
without falling prey to the criticisms that were
fatal to the other theories of truth.
Skepticism about Skepticism
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Heraclitus maintained that identity over time
is an illusion. He thought that the world was
being created anew each instant.
“You cannot step into the same river twice,”
he proclaimed, “for the water into which you
first stepped has flowed on.”
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Parmenides thought that reality must
contain a continuing substance, because
only that which is unchanging is real.
He reasoned that nonexistence
(nothingness) cannot exist.
Therefore, everything that exists must
always have existed.
From nothing, nothing comes, and
nothing cannot come from something.
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Paramenides believed that because
nonexistence cannot exist, it cannot be
thought about.
Do you agree? Can you think about
nothing? That is, can nothingness be the
object of your thought?
If so, can you describe what you’re
thinking about when you’re thinking
about nothing?
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In this stadium, before
you reach the door you
must reach the point
halfway there. But
before you reach the
halfway point, you
must reach a point
halfway to that.
Since it takes some
finite interval of time
to move from one
point to another, and
there are an infinite
number of halfway
points, it would take
you an infinite time to
pass through them all
and get out.
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Plato, following
Parmenides, held that
only that which is
unchanging is real, and,
following Heraclitus, that
what is presented to our
sense is constantly
changing.
He concluded that what
we sense isn’t fully real.
The prisoners in the cave
see only the shadows
cast by the truly real
objects: the “forms.”
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British philosopher Nick Bostrom thinks there
is a good probability that we’re living in a
computer simulation.
Do you think that Bostrom’s simulation
argument is a good one? That is, do you think
it is probable that you are living in a
computer simulation? Why or why not?
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Do you think that there are any concepts or
truths that all normal humans have or know?
If so, what are they?
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Descartes doubted that sense experience
can give us knowledge because knowledge
requires certainty and nothing we learn
through our senses is certain.
Descartes holds that you are justified in
believing something to be true only if you
are certain of it.
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How often has it happened that you thought
something was real only to find out that you
were dreaming?
Can you know for certain that you’re not
dreaming right now?
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Suppose that every dream you had ended by
your getting into bed and going to sleep.
Would you be able to tell what was a dream
and what was reality? How?
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We can’t be certain that we’re not
dreaming.
If we can’t be certain that we’re not
dreaming, we can’t be certain that what
we sense is real.
If we can’t be certain that what we sense
is real, we can’t acquire knowledge
through sense experience.
Therefore, we can’t acquire knowledge
through sense experience.
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Suppose that an evil genius used
telepathy (or a mad scientist used
electrodes) to put false ideas into your
mind.
Can you be certain that you’re not under
the influence of such a person right now?
If not, can you have knowledge of the
external world?
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We can’t be certain that our sense
experience is not caused by an evil
genius.
If we can’t be certain that our sense
experience is not caused by an evil
genius, we can’t be certain that what we
sense is real.
If we can’t be certain that what we sense
is real, we can’t acquire knowledge
through sense experience.
Therefore, we can’t acquire knowledge
through sense experience.
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Descartes cannot doubt that he is thinking,
for doubting is a type of thinking.
And Descartes can’t doubt anything unless he
exists.
So Descartes claims that he can be absolutely
certain of one thing, namely, “I think,
therefore I am.”
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Descartes knows the contents of his mind; if
he seems to see a tree, he knows that he
seems to see a tree.
To acquire knowledge of the external world,
he needs a principle to bridge the gap
between appearance and reality.
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God exists and is no deceiver.
If God exists and is no deceiver, then
whatever I clearly and distinctly perceive is
true.
Therefore, whatever I clearly and distinctly
perceive is true.
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I clearly and distinctly seem to see a
tree in front of me.
Whatever I clearly and distinctly
perceive is true.
Therefore, there is a tree in front of
me.
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Descartes can’t know that God exists and is
no deceiver unless he knows that what he
clearly and distinctly perceives is true.
But he can’t know that what he clearly and
distinctly perceives is true unless he knows
that God exists and is no deceiver.
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Foundationalism maintains that there
are basic beliefs, that is, beliefs whose
justification does not depend on other
beliefs.
Also, it maintains that the justification
of all other beliefs depends, at least in
part, on the basic beliefs
The problem of the Cartesian circle
faces anyone who takes a
foundationalist approach to knowledge.
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The fact that we ordinarily claim to know
many things that aren’t certain casts doubt
on the claim that knowledge requires
certainty.
But if it’s doubtful that knowledge requires
certainty, then Descartes can’t know that
knowledge requires certainty.
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To know a proposition, it doesn’t have to be
established beyond a shadow of a doubt. It
only has to be established beyond a
reasonable doubt.
This is the standard that is used in courts of
law to adjudicate matters of life and death; it
can also be used to adjudicate matters of
knowledge and ignorance.
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Empiricism leads to skepticism about
anything that cannot be sensed.
Empiricists such as David Hume believe that
only terms that stand for ideas derived from
sense experience can refer to real objects.
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Induction assumes that the future will
resemble the past.
What justifies our believing that?
If this belief must be accepted on faith, is
science a religion?
Does hypothetical deduction (inference to
the best explanation) suffer from the
same problem?
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Kant noticed that some truths can be
known without being empirically
confirmed, but also are not true by
definition.
According to Kant, to make sense of the
world around us, we naturally categorize
the world into a conceptual scheme.
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Are scientific laws invented or discovered?
The traditional view is that scientific laws
exist “out there” in the world and that the
job of the scientist is to discover them.
Kant, however, claims that “the order and
regularity of the appearance we entitle
nature, we ourselves introduce.” For him,
reality is a human construct.
Which view do you think is correct? Why?