PHIL 111: Introduction to Philosophy

LARKIN: Spring 2003

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Descartes, Meditation One

 

I.                     Cartesian Project

A.     Quest for Certainty

1.  Descartes has realized that he has some bad beliefs.

2.  This raises suspicion about whole belief system—if some are bad, then others might be too, plus many beliefs have since been built on the bad ones.

3.  Descartes wants a system that does not have any false beliefs and where all the beliefs are certain/guaranteed.

4.  Foundationalism: Our belief system is built on a foundation of basic beliefs, which can justify other beliefs but are themselves not justified by other beliefs.

5.  Certainty demands finding basic beliefs that are absolutely certain and connections between beliefs that preserve that certainty.

 

B.      Method of Universal Doubt

1.  Descartes wants to knock down his entire old system of beliefs.

2.  So he wants to generate doubt about all of his previous beliefs, or as many as possible.

3.  Descartes wants to find certain foundations on which to build a new system of beliefs.

4.  So he wants to actively try to doubt everything in order to find something that he cannot doubt—something that is indubitable and therefore certain.

5.  Thus Descartes embarks on a process of trying to doubt as many beliefs as possible by formulating skeptical arguments.

 

 

II.                   Skeptical Arguments

A.      Unreliability of the Senses Argument

1.  The argument

P1:  My senses sometimes deceive me.

 

P2:  One should never completely trust what once has deceived one.

 

C1:  So I should not completely trust my senses.

 

P3:  All of my beliefs have come either from or through my senses.

 

C2:  So I should not completely trust any of my beliefs.

 

2.  Problem:  The argument is self-defeating

If the conclusion is true, then the first premise is undermined.  For we cannot know that our senses sometimes deceive us unless we at some point trust our senses to tell us that our senses at some other point were mistaken.

 

3.  Works for:

The argument succeeds in generating doubt about empirical propositions concerning ‘extreme’ circumstances—objects that are far away, very small, dimly lit, etc.

 

4.  But not for:

The argument does not generate doubt concerning propositions about matters ‘close at hand’.  Our senses do sometimes deceive us, but not about matters in ordinary closes-at-hand circumstances.

 

 

B.      Dream Argument

1.  The argument

P1:  If I really know that P, then I can rule out the possibility that I am merely dreaming that P.

[If I cannot tell whether I am dreaming, then I cannot really tell whether there is a table in front of me.  If I was merely dreaming the table, then there (probably) would not really be a table in front of me.]

 

P2:  But I cannot rule out the possibility that I am merely dreaming that P.

[There does not seem to be any test that I can perform that I can use to definitively determine whether I am dreaming or not.  For any test we come up with, it seems we can dream that we perform it and dream that we get whatever results.]

 

C:  So I do not really know that P.

 

2.  Argument Form

a.        The argument form exemplified here is called modus tollens or denying the consequent.

P1:  If P, then Q.

P2:  Not-Q.

C:  So Not-P.

 

b.       Denying the Consequent is valid:  It is impossible for the premises to be true but the conclusion false.

c.        So if the conclusion is false, at least one of the premises must be false.

d.       This form should be carefully distinguished from the following, called denying the antecedent:

P1:  If P, then Q.

P2:  Not-P.

C:  So, Not-Q.

e.        Denying the Antecedent is Invalid.

f.         It is possible for the premises to be true but the conclusion false:

P1:  If Lassie is a dolphin, then she is a mammal.

P2:  Lassie is not a dolphin.

C:  So Lassie is not a mammal.

 

 

3.  Works for:

The Dream Argument is sufficient to generate doubt for propositions about complex objects—like tables and chairs and bodies and such—the kinds of things studied by Physics, Astronomy, and Medicine.

 

4.  But not for:

The Dream Argument is not sufficient however to generate doubt for propositions about simple objects—like colors, shapes, quantities, space, time—the kinds of objects studied by Arithmetic and Geometry.

 

 

 

 

C.      Evil Genius Argument

1.  The Argument

P1:  If I really know that P, then I can rule out the possibility that there is an all-powerful evil genius deceiving me about P.

 

P2:  But I cannot rule out the possibility that there is an evil genius deceiving me.

 

C:  So I do not really know that P.

 

2.  Works for:

The Evil Genius argument is the best possible skeptical argument—the evil genius is all-powerful and so can generate doubt about anything for which it is possible to generate doubt about.  The argument works for propositions about complex objects as well as propositions about simple objects.

 

3.  But not for:

Still the Evil Genius argument cannot generate doubt about my own existence or the fact that I am thinking.  Not even an all-powerful evil genius can make me think that I exist when I do not exist.  This would require making me exist and not exist at the same time, which is impossible.  And not even an all-powerful evil genius can make me think that I am thinking when I am not thinking.  This would require making me think and not think at the same time, which is impossible.