Transcript Descartes Meditations

Descartes’ Meditations
Perceptual Knowledge
The Goal
Descartes wanted certainty.
•
How can we know, for sure,
that science is on a sure
footing?
•
How do we guard against
discovering, centuries from
now, we were wrong about
basic observations on which
science is built?
•
Can we improve the
foundations of our knowledge?
The Method of Doubt
Descartes employs a conceptual tool to find the
certainty he seeks: the method of doubt.
•
•
If it is possible to doubt something,
Descartes will reject it completely.
If it is impossible to doubt something,
Descartes will accept it as a cornerstone for
science, to see what can be built on it.
But, can’t we trivially doubt everything?
Yes, but we cannot methodologically doubt
everything. To methodologically doubt
something, we have to think of how it
might be false.
First Meditation*
Method to Doubt the Senses
1. Variability of Sensation Argument:
Our senses deceive us:
when we see something far off, our judgments are often
wrong
when we see very small things, our judgments are often wrong
We shouldn’t trust anything that has deceived us in the
past.
*This is a nice place to read Descartes’ Meditations
First Meditation
Method to Doubt the Senses (cont.)
2. Dream Argument:
But what about trusting our senses
regarding things close up, like the
fire in the fireplace, and our own
bodies?
“Often in my dreams,” says Descartes,
“I am convinced of just such
familiar events – that I am sitting
by the fire in my dressing-gown –
when in fact I am lying undressed
in bed!”
First Meditation
Method to Doubt the Senses (cont.)
2. Dream Argument (cont.):
When we are dreaming, nothing we see or hear or feel is
real.
Also, there are no sure signs by which to distinguish
waking from sleeping.
“As I think about this more carefully, I realize that there is
never any reliable way of distinguishing being awake
from being asleep.”
First Meditation
Method to Doubt the Senses (cont.):
Okay, so, now we have lost the world of sense perception.
Still, Descartes reminds us,
“For whether I am awake or asleep, two plus three makes five, and
a square has only four sides.”
And the simple qualities that make up the objects in dreams still
retain their reality.
At least we still know those things, even if we are dreaming!
Except …
First Meditation
Method to Doubt Reason Itself!
“I have for many years been sure that there is an all-powerful God....
How do I know that he hasn’t brought it about that there is no
earth, no sky, nothing that takes up space, no shape, no size, no
place, while making sure that all these things appear to me to
exist? …how do I know that I myself don’t go wrong every time I
add two and three or count the sides of a square?”
But perhaps there is no God? Descartes replies:
“But the less powerful they make my original cause, the more likely it is
that I am so imperfect as to be deceived all the time….”
First Meditation
Method to Doubt Reason Itself!
Never mind atheism. I forgot my project is to destroy everything
I can with the strongest methodological doubt, to see what
remains, if anything.
“I shall suppose that some malicious, powerful, cunning demon
has done all he can to deceive me – rather than this being
done by God, who is supremely good and the source of
truth.”
So, note that this evil demon can do what God can do: make
Descartes fail to properly add 2+3, or count the sides of a
square … and, being evil, will do so.
Descartes’ point: since this is possible, he has no certainty of
even reason itself!
Second Meditation
Method to Doubt Reason Itself Leads
to One Certainty:
“But there is a supremely powerful and
cunning deceiver who deliberately
deceives me all the time! Even
then, if he is deceiving me I
undoubtedly exist: let him deceive
me all he can, he will never bring it
about that I am nothing while I
think I am something. So after
thoroughly thinking the matter
through I conclude that this
proposition, I am, I exist, must be
true whenever I assert it or think
it.”
“Dubito ergo cogito, cogito ergo sum.” –Descartes, Discourse on Method
Second Meditation
Method to Doubt Reason Itself Leads to One Certainty:
So, if an evil demon is deceiving you, you have to be there
to be deceived.
Descartes has found that one indubitable truth he sought.
This first indubitable truth
guarantees 2 things:
1. You know at least one
thing is true
2. You know at least one
thing exists
Second Meditation
Okay, so, Descartes knows he exists. But, what is he?
Does he know he has a body?
Does he know himself to be a spirit or fire or breath or ether?
“Well, then, what am I? A thing that thinks. What is that? A thing
that doubts, understands, affirms, denies, wants, refuses, and
also imagines and senses.”
So, Descartes is a thing that thinks. What can he infer from
that? That he also doubts, understands, affirms, etc? He
knows those more directly than that he thinks, since
thinking is the genus of those species.
Third Meditation
So, can Descartes get any further than just his own existence as a
thinking thing?
What can be built on that foundation, that cornerstone?
“I am certain that I am a thinking thing. Doesn’t that tell me what
it takes for me to be certain about anything? In this first
item of knowledge there is simply a clear and distinct
perception of what I am asserting; this wouldn’t be enough
to make me certain of its truth if it could ever turn out that
something that I perceived so clearly and distinctly was false.
So I now seem to be able to lay it down as a general rule that
whatever I perceive very clearly and distinctly is true.”
Third Meditation
Are our beliefs about the external, physical world clear and
distinct in this way? Descartes says, no.
Are our beliefs that 2+3=5, or a triangle has 3 sides clear and
distinct in this way? Descartes says, no.
Why?
Because of that possible evil demon!
“… when I get the opportunity I shall examine whether there is a
God, and (if there is) whether he can be a deceiver. If I don’t
settle this, it seems, then I can never be quite certain about
anything else.”
Third Meditation
What about the principle of sufficient reason, is it clear
and distinct in this way? Descartes says, yes.
“Now it is obvious by the natural light that the total cause
of something must contain at least as much reality as
does the effect. For where could the effect get its reality
from if not from the cause?”
Then later,
“The longer and more carefully I examine all these points,
the more clearly and distinctly I recognize their truth.”
Third Meditation
At this point, Descartes presents quite a complex argument
for the existence of God, based on the principle of
sufficient reason and the existence of a clear and
distinct idea of God in his mind.
Rather than evaluate the argument, which is historically
underwhelming, let’s just acknowledge the move and
look at the whole picture of Descartes’ strategy.
Descartes’ ‘Surprising Circuit’
David Hume wrote that Descartes’ answer to skepticism, or
skeptical doubts, involved a ‘surprising circuit’. Here it
is:
Doubt
Method of Doubt
Certainty of ‘I exist’
Certainty of God’s Existence
Certainty of God’s Goodness
Confidence of External World
Foundationalism
Descartes’ philosophy nicely illustrates the epistemological
position of Foundationalism.
Foundationalism: knowledge is to be understood as two
tiered:
•
there is basic, foundational knowledge, knowledge
based not on evidence but on what is evident, that
forms the foundation of all other knowledge, and
•
non-basic knowledge, knowledge derived logically from
those basic pieces of knowledge
*An alternative account of the structure of knowledge, for the sake of contrast, is
Coherentism (knowledge consists in the compatibility and mutual-support among
some maximally consistent set of our beliefs, none of which is evident, but all of
which is supported by the coherence of the whole web).
Foundationalism
Questions:
What is Descartes’ first, most basic piece of
foundational knowledge?
How does Descartes extend his knowledge from
that first basic piece?
Once Descartes proves God’s existence and
concludes he can rely on his goodness not to
deceive him, what much wider foundation for
knowledge is available to him?
Without knowledge of God’s existence, is there
any hope of defeating the evil demon of
skepticism?
How can you get knowledge of God’s existence
without first defeating the evil demon of
skepticism?