A posteriori Knowledge
A priori knowledge
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A posteriori knowledge is based on
experience.
A priori knowledge is knowledge that
is not based on experience.
If you can know something just by
thinking about it, without consulting
how the world appears to you, that is
a priori knowledge.
Possible examples of a
priori knowledge
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“bachelors are unmarried”
2+2=4
There cannot be a round square
Any logical truth—the connection
between premises and conclusion in a
valid argument
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The dream argument calls into
question knowledge based on
experience, but it leaves a priori
knowledge untouched
But the evil demon argument
questions also a priori knowledge
The EVIL DEMON!
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It is possible that there is a demon,
with nearly godlike powers, who aims
to deceive me.
Therefore it is possible that all my
beliefs, are the result of the deceptive
powers of a demon
Therefore I cannot trust any of my
beliefs
Meditation 2
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Descartes now tries to discover some
belief or beliefs that can be known
with certainty. These beliefs would
have to be true even if there is a
demon
He finds one: I exist.
But could an evil demon deceive him
(or you) about your own existence?
Descartes answers no. Deception
requires that there be something to be
deceived.
Even if all my thoughts are mistaken,
the thoughts must exist in order for
them to be mistaken
Everytime I reflect on my thinking I can
know with absolute certainty that I
exist.
But what sort of thing is
this “I”
Am I a rational animal?
 Am I a human being?
 What is essential to this “I” that I
know to exist?
Essentially, what I am is not human
being, not rational animal, but a
thinking thing
Why?
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The knowledge that I am human or an
animal, or even have any body, is still
in doubt. A demon can deceive me
about these things. So I cannot know
anything about my physical being. But
there is something I know. What I
know is the residuum, it is what is left
over after all that can be doubted is
doubted—thinking, consciousness.
Solipsism!
Solipsism is a view that can only be
stated in the first person.
 My mind is all that exists, everything
else only exists as I experience it.
There are few if any solipsists. But is
there any way to refute solipsism?
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The piece of wax
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Therefore we know the mind with
much more clarity than we know the
body.
But this is hard to believe. Its easy to
think that knowledge of the physical
world is much more concrete,
“tangible,” and that the mind is
somehow mysterious
The piece of wax example shows us
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Take a piece of wax, melt it.
All the sensible properties change
Yet we still believe, we still take the
wax to be the same thing
This shows: [supposed] knowledge of
physical things is based on the mind.
We need to think to know the physical
world
Our concept of the physical world is
not a bundle of sensible qualities.
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What we understand when we
understand that the piece of wax is
the same is a “flexible, extended
substance”
It is what underlies or causes our
perceptions, not what we directly
perceive
Descartes is foreshadowing future
conclusions in the Meditations.