Transcript RATIONALISM – CARTESIAN MOTIVATIONS

René Descartes (1596-1650)
• Father of modern rationalism.
• Reason is the source of knowledge, not
experience.
• All our ideas are innate.
• God fashioned us with these ideas.
• We discover basic truths by intuition: by
grasping basic connections between the ideas
we have.
• We deduce or demonstrate more complex
truths.
The Scientific Revolution
• Lived during the Scientific Revolution.
(17th century).
• This was a time of great intellectual change. Most
famously, people started to believe that the earth orbited
the sun thanks to Copernicus (in 1543).
• Descartes’ predecessors believed that knowledge begins
with experience (empiricism)
• Descartes argued that this view must change too.
• His Meditations (1641) is effectively a manifesto for
rationalism.
Aim and method
• Descartes wants knowledge.
• He knows that he has many false beliefs.
• He needs to weed them out to establish base of indubitable,
necessarily true beliefs.
• Foundationalism – basic beliefs provide the ultimate source of
justification.
• His method is to challenge each thing he believes to see whether it
is “completely certain and indubitable”. This is known as the method
of doubt.
• Key reading – Ch. 4; Meditation 1.
Scepticism: Wave I
The basis of our ordinary
beliefs is sensory
experience.
But we all know that our
senses are fallible.
The pencil is not really
bent.
But that our senses
can let us down
sometimes is no
good reason to
believe they always
do. I would be mad
to deny that I am
sitting here at my
desk looking at a
screen!
Scepticism: Wave II
But what if I were dreaming?
If there are
no clear
signs by
which to
distinguish
waking
experiences
from dream
experiences,
then I
cannot be
certain that
any sensebased belief
is true.
And whether I’m
dreaming or awake,
2+2=4 and a square
has four sides.
But dreams are like paintings. A
painting may be of something that
does not exist (like a winged
badger) but the raw ingredients –
colours and shapes – must.
In the same way,
there must be colours
and shapes I have
experienced in order
to create dreams. So I
can be certain that
there is a world out
there even if I can’t be
sure I’m experiencing
it now.
Scepticism: Wave III
But what if I were in the grip of all-powerful demon
– like the Matrix.
He could not
just delude
me into
thinking I
have a
body…
…but
deceive me
about
everything!
The existence of space and the
‘truths’ of mathematics and
geometry could all be powerful
illusions!
If only I could find
just one thing of
which I could be
certain!
If so, I cannot be
certain of anything.
The sceptic has
shown that
philosophy’s goal of
obtaining certainty
cannot be met.
Descartes and the Cogito
“So after considering everything very
thoroughly, I must finally conclude that this
proposition, I am, exist is necessarily true
whenever it is put forward by me or
conceived in my mind.” Meditation II.
Cogito, ergo sum
I think, therefore I am
Descartes’ first certainty is established
through the application of reason alone: I
exist. He has put into doubt all beliefs
arrived at by the senses through the use of
the ‘evil demon’. His certain existence results
from his awareness of himself as the subject
of his thoughts.
Descartes and the Mind
I know that I exist.
But what am I?
I am a thinking thing –
a non-physical
conscious self
Descartes argues that he can clearly doubt or
conceive of himself lacking a body whilst still
existing but not a mind. So, he and his body
are distinct things.
It is reason that tells him that he exists and what he is.
Descartes and the wax I
BEFORE
Sight: bright yellow
Smell: flowers
Taste: pollen
Sound: dull thud
Feel: solid
If all I had were senses, I
would have to conclude the
old wax had been destroyed
and that something new had
appeared….
…but I know it is the same wax.
AFTER
Sight: dull yellow
Smell: none
Taste: none
Sound: none
Feel: soft, malleable
Descartes and the wax II
How do I know it is the
same wax if my senses
tell me that everything
about it has changed?
Perhaps my power of
imagination – my
ability to create mental
pictures – is at work.
I can picture all the ways
the wax could look and
hence am able to track the
wax as the same wax.
This can’t be right. There are far too
many ways the wax can appear for
me to have imagined them all.
I can’t imagine 80 dots in a line and
picture them dividing into two groups…
Furthermore, my powers of
imagination are extremely limited!
…but I can think of 80 dots and divide
them into two groups using my powers
of reason.
Descartes and the wax III
What enables me
to see the wax is
my mind – reason.
Reason understands
that the essence of
material things is
extension: being
(filled) bits of threedimension space.
I thus have the idea of a 3D
object. My senses present me
with the surface appearances
and reason makes me see
them as features of a persisting
object.
I say, “I see the wax!”
In reality, I see the colours and judge
that there is wax there.
In the same way, I say, “I see the
men in the square.”
In reality, I just see clothes but I
judge that there are men (and not
automatons) underneath.
Descartes and God: The Ontological Argument
We understand the concept of a
unicorn. We grasp the essence of a
unicorn as a magical white horse
with a horn.
…omnipotent:
maximally powerful.
Except in
the case of
God. God is
an infinitely
perfect
being. He
is…
…omniscient: maximally
knowledgeable.
Essence is one thing and existence
another. Just because the idea of a
unicorn makes sense, it doesn’t mean
there are any.
4:
3:
2:
1:
…omnibenevolent:
maximally good.
…necessarily existent:
maximally real.
necessarily existent: God
contingently existent: you
contingently non-existent:
unicorn
necessarily non-existent:
round square.
So if it is part of God’s
essence to exist necessarily,
God must exist!
The naïve view of our perception of the world
The redness
in the cube is
transmitted to
my mind
where I
perceive it
Red light
travels to my
eye.
Redness is in
the cube. It’s a
feature of the
world.
Descartes and our perception of the world
Colourless matter:
Our innate ideas
of colours are
triggered in the
mind by the
information in the
brain sent from
the eyes. The
colour is in me.
Colourless signals
are sent to my brain.:
Colourless matter:
Colourless light is
sent to my eye.:
The cube is red in the
sense that it has a
chemical structure
that our brains react
to with a sensation of
red. It is not red in the
way we experience it
as red.
Descartes’ Rationalist Conclusions.
The senses are
fallible and could be
deceived.
Cogito, ergo sum: I
think, therefore I am.
My ability to think
proves I exist.
Reason is needed for science.
Science is not concerned with
the sensible properties of
things. It is concerned with
quantifiable properties.
Reason is required to work out
the mathematical structure of
the universe: e.g. the laws of
physics.
And all the ideas I have are
innate!
Reason proves that
God exists too.
Reason leads me to
the conclusion that I
am a thinking thing.
It is also thanks to
reason that I see the
world. The senses
inform us of a
changing world.
Reason grasps the
underlying constancy
of the world. It
enables us to see
things. (Think of the
wax example.)