Descartes is skeptical about the truth of one’s beliefs. He develops the following argument:

1) If I am to know that I am sitting by the fire (P), then I must know that I am not dreaming that I am sitting by the fire (Q)
P → Q 
2) I cannot know that I am not dreaming that I am sitting by the fire (∼ Q)
Therefore, I do not know that I am sitting by the fire (∼ P).

P → Q 
∼ Q 
— 
∼ P

The possibility that Descartes might be dreaming really threatens to his knowledge of the world around him. He is right in thinking that he must know that he is not dreaming if he is to know something about the world around him. He is also right in his discovery that he can never know that he is not dreaming since evidence of not dreaming acquired through dreams is not admissible. The evidence should be legitimate. Thus, it is entirely possible that we can know nothing about the external world.

Bibliography:

  • Descartes’ Meditations in Modern Philosophy: An Anthology of Primary Sources, Roger Ariew & Eric Watkins (eds.), Hackett Pub. Co. (2009)