Philosophy 210/310
Early Modern Philosophy 
Andrew Mills' 
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Two Arguments from Meditation I
(from Georges Dicker, Descartes, with some modifications)

The Deceptiveness of the Senses

  1. If my senses sometimes deceive me, then it’s possible that my sense always deceive me.
  2. My senses sometimes deceive me.
  3. So, it’s possible that my senses always deceive me. (from 1,2)
But, Descartes reflects, (2) is true only in “suboptimal” situations: when I am tired, or have a cold, or drunk, or without my glasses, or when it is too dark, or the object is too far away, etc.   If this is right, then the conclusion that is warranted is only this:
It’s possible that my senses always deceive me in suboptimal perceptual situations.
We still have no reason to doubt our senses in optimal situations (full light, wide awake, sober, etc.)  In none of those situations have my senses ever deceived me, so I have no reason to doubt them now.  It is here that Descartes turns to the Dream Argument.

The Dream Argument

  1. I sometimes have vivid dreams that are qualitatively just like my “best” waking perceptions.
  2. If I sometimes have vivid dreams that are qualitatively just like my “best” waking perceptions, then I cannot distinguish with certainty between my “best” perceptions and vivid dreams.
  3. I cannot distinguish with certainty between my “best” perceptions and vivid dreams. (from 1,2)
  4. If I cannot distinguish with certainty between my “best” perceptions and vivid dreams, then even my “best” perceptions provide no certainty.
  5. So, even my “best” perceptions provide no certainty (from 3,4)
One worry is that this argument is self-refuting.  If (3) is true, then (1) cannot be true.  For to assert (1), one must be able to distinguish between waking perceptions and vivid dreams, but (3) says precisely that this cannot be done.