Descartes
Meditations
Knowledge needs a foundation
Descartes knows he has false beliefs, but he
does not know which ones are false
So, we need a method to sort out the false
beliefs from the true ones
The Method of Doubt
Treat all beliefs that are not absolutely
certain as if they were false.
Try to determine if there is anything that
can be known with absolute certainty, any
belief that is immune to doubt.
You don’t need to go through all your
beliefs one by one to see if they are
doubtful.
You can show a belief is doubtful if you
can show that its source is unreliable
What is the source of most of our beliefs?
Doubting the senses
Sense experience is the source of most of
our beliefs
So if we can show that sense experience
is doubtful, then we can show that all the
beliefs based on sense experience are
doubtful.
The Dream Argument
When we dream we have a sense
experience.
The objects of dreamed sense experience
are unreal (dreams are a deceptive sense
experience)
If the senses deceive us in dreams, they
can deceive us any time
Therefore, all the beliefs based on sense
perception are in doubt.
A priori and A posteriori Knowledge
A posteriori knowledge is knowledge
based on the senses.
A priori knowledge is knowledge that is not
justified on the basis of sense experience
If a belief is justified no matter what sort of
sense experience you have, then it is a
priori.
Possible Examples of A priori
Knowledge
There cannot be a round square
2+2=4
The sum of the angles of a triangle is 180
degrees
All bachelors are unmarried.
Nothing can be blue all over and red all
over at the same time in the same respect.
Descartes: mathematical and geometrical
truths can be known even if I am living a
huge consistent dream/hallucination.
Therefore these are a priori and not called
into doubt by the dream argument
The Evil Demon!
It is possible that there is a very powerful
deceiver, a being that aims to deceive me
about everything I believe.
Therefore it is possible that everything I
believe is false.
This argument applies to a priori and a
posteriori beliefs. The demon can mess
with how you think, as well as what you
perceive.