[Western Oregon University]



by Dale Cannon


     1. Descartes is a rationalist who set out to refute radical skepticism on its
         own turf.

     He sought an absolute foundation for knowledge by proposing to doubt
     all things and accept as knowledge (or at least as a foundation for
     knowledge) only what could not be doubted.  (Note that this requirement
     of absolute certainty [undoubtability] was not Plato's or Aristotle's
     criterion for knowledge.)

     2. Descartes' procedure is to withhold his belief from anything that is not
         entirely certain and indubitable.

     This leads him to consider the possibility that instead of a benevolent God,
     there is a powerful and evil demon systematically deceiving him into
     thinking things to be so that are not in fact so.  This leads him to conclude
     as doubtable, and therefore as not knowledge,
          sense experience, and all that sense experience testifies to (e.g., that
          there is an external world, other people, and even that he has a
          his conviction that what he takes to be waking reality is real and not a
          dream (or a cosmic deception),
          his memory, and
          intellectual calculation (e.g., 2 + 3 = 5).

     3. The one thing Descartes finds to be absolutely certain in the midst of
         radical doubt and possible deception is that thinking (especially in the
         mode of doubt) exists, that he as a thinking thing exists.

This will become Descartes' foundational truth and the measure of all other truth:  Cogito [I think], ergo [therefore] sum [I am].
      4. From there Descartes investigates, solely on the basis of dialectical
          reasoning apart from reliance upon what has proved to be doubtable,
          and concludes

          a.  What must be the criterion of knowledge -- namely, a candidate for
          belief whose certainty is wholly evident to the reflecting mind with
          the "clarity and distinctness" of the cogito's existence to itself;
          b.  What his essential nature must be -- namely, a thing that thinks
          (including also doubts, understands, affirms, denies, wills, refuses,
          imagines, and feels); and
          c.  What, to the contrary, must be the essential nature of the bodies to
          which our senses uncertainly testify -- namely, things which are
          extended in space.

      5. Descartes goes on to extend his foundation for knowledge and show
          how it can provide a basis for the general trustworthiness of sense
          perception, memory, and intellectual calculation, among other things,
          by offering what he believes to be proof of the existence and goodness
          of an infinitely powerful, wise, and good creator of himself (as a finite
          and fallible mind), a creator whose goodness would never allow his
          creature to be comprehensively deceived.

     6. Thus Descartes believes he has provided a foundation, on the one
          hand, for knowledge in morality and religion (in the mind's or soul's
          givenness to itself) and, on the other hand, for knowledge in the natural
          sciences (in the nature of physical bodies to which the senses give us


     1. Error results because we tend to jump to conclusions, make judgments
         too quickly on imperfect evidence.  I.e., we make mistakes because we
         do not exercise our capacity to suspend judgment: we assent or
         dissent before we have enough evidence.

     2. We can avoid mistakes and achieve certainty if we hold back and
         assent only to those propositions which we clearly and distinctly
         perceive to be true (i.e., propositions for which we have fully sufficient

     3. When we confront a statement for which there is no compelling
         evidence either that it is true or that it is false (where the truth is not
         perceived with sufficient clarity and distinctness), we should suspend
         our judgment until we have compelling evidence.

     4. We shall unquestionably reach the truth, if only we give sufficient
         attention to all the things which we perfectly understand, and separate
         these from all the other cases where our apprehension is more
         confused and obscure.

     5. Every clear and distinct perception must have God for its author and
         thus it cannot be deceptive, because God is no deceiver.


     1. Insistence upon a standard of absolute certainty eliminates the middle
         ground of reasonable evidence.  It suggests that if you don't have
         complete certainty you have no evidence at all.

     2. Insistence upon absolute clarity and distinctness to the skeptical
         reflecting mind eliminates consideration of any respect in which reality
         transcends full and determinate representation.

     3. Proposing to rebuild one's knowledge from the ground up because a
         number of things that once seemed true have become doubtful or false,
         as Descartes does, is a lot like being in a boat out on the ocean and
         proposing to abandon ship in order to rebuild the boat from the keel up
         just because it has developed a few leaks.

     4. Descartes' working method of radical doubt may create more problems
         than he supposes.  In particular, it seems to put one in a very peculiar,
         abstracted state of mind where the usual clues on which we rely to
         orient ourselves and make sense of things are no longer available.  In
         other words, it may serve to cut us off from reality rather than put us
         more closely in touch with reality.

     A. It disables reliance upon methodological faith, such that knowledge
          by acquaintance (whether in sense perception or in reasoned
          intuition) seems no longer trustworthy.
     B. It makes experience seem passive, surface only, without depth and
          atomistic (non-integrated).
     C. It cuts the mind (and all internal mental life) off from its sense of being
          in the world, being in context, situated in a time, place, culture,
          language, and in rapport with other people -- also from learning
          through empathy.


     1. If absolute certainty is not the standard or criterion of knowledge, then
         what is the standard?

     Human claims to knowledge are fallible.  They can be wrong.  To make a
     claim to knowledge is to risk being wrong.  To make a claim to knowledge
     is like making a wager or, alternatively, like making a promise, giving one's
     word, that something is so.

     Thus, there is always some room for doubt.  The criterion is to eliminate or
     answer reasonable doubt, through having and offering sufficient
     evidence.  (See the handout on the nature and criticism of arguments.)
     What sufficient evidence is will vary with the context and type of
     knowledge being claimed.

     2. But that won't convince the skeptic, will it?

     The skeptic needs to be convinced only if
        (a) he is being reasonable in his doubt,
        (b) he is really interested in getting at the truth in question, and
        (c) he is truly open to weighing relevant reasons.
     If he is not, his opinion ceases to count in reasonable argumentation until
     he meets these conditions.


     Descartes' goal: to refute the possibility that there could be an all
     powerful, demonic higher power (an "evil genius") systematically
     deceiving us (with regard to the general trustworthiness of sense
     perception and the reliability of reason).

     A. We have a clear and distinct idea of God.

     B. This is the idea of a perfect being, an infinite substance, eternal and
          unchanging, supremely intelligent and powerful, existing
          independently -- a being which created ourself and everything else
          [whatever else exists].

     C. Something cannot arise from nothing.  (I.e., everything must have a

     D. A cause must contain at least as much reality (as much perfection) as
          its effect.  (I.e., what is more perfect cannot arise from what is less

     E. Being an imperfect and finite (limited) being, we cannot ourself produce
          this idea of a perfect being.
     F. Therefore, God must exist as the being who has produced in his
          creature, namely us, the idea of a perfect being (God himself.)

     G. All fraud and deceit depend upon some defect.
     H. Therefore, God cannot be a deceiver.

     I. The only reason for doubting that human reason is reliable is the
        possibility of a higher power deceiving us (the Evil Genius hypothesis).
     J. Therefore, human reason is reliable.

     [A problem:  Doesn't this proof assume that human reason is reliable, and
     thus assume the very thesis it is trying to prove?  I.e., does it not "argue in
     a circle"?]


     Descartes' goal: to establish a firm foundation on the basis of which
     natural science can proceed to achieve an understanding of things that
     will make us "masters and possessors of nature."

     1. Natural science seeks knowledge of the external world.
          a. The external world is made up of extended things, bodies in space
          -- and determinately representable in mathematical form.

          b. The only basis we have of knowledge of the external world is
              through sense perception.

          c. Sense perception, while fallible, is on the whole reliable because
              the all powerful being who created us is no deceiver.

          d. But this requires that we be very circumspect in relying on the
              evidence of sense perception.

              Specifically we must rely only on evidence which is exactly
              specifiable (ideally in mathematical form), and invariant from
              person to person and under varying conditions of perception
              -- in short, impersonally given.
     2. Natural science must therefore be very careful and discriminating of
         how it relies on sense perception.  As revealed through the method
         of systematic doubt, sense perception is at best a representative
         appearance, within the mind, of what is outside the mind.
          a. Sense perception is conceived to be a product of the causal
              impact of features of things in the external world upon the sense
              organs of our body.

          b. This impact produces two kinds of apparent features or qualities
              in our sensory impressions:

              i.  Impressions which more or less accurately represent the
                  features of bodies in the external world ( = primary qualities), and

              ii.  Impressions which do not correspond but which are simply
                   produced in us as a result of the impact of these features on
                   our sense organs ( = secondary qualities).

          c. Secondary qualities are conceived to vary from person to person
              and underdifferent conditions of perception.  This makes them
              unreliableindicators of the true nature of things ( = subjective).

             i.  Examples include color, texture, warmth, meaning, value, and
                 purpose -- i.e., all qualitative features.

          d. Primary qualities alone are conceived to be indicators of the true
              nature of things( = objective), and they are what natural science
              must solely relyupon.

              ii.  Examples include size, shape, mass, location, and motion -- i.e.,
                   all quantitative features of extension.

Western Oregon University
Copyright © 1997 Western Oregon University
Direct suggestions, comments, and questions about this page to Dale Cannon.
Last Modified 5/6/01