A little more needs to be said. Descartes' original intention was to build a new system of philosophy, without incorporating any, potentially flawed, ideas from his previous beliefs. So he began his method of systematic doubt. A process he actually borrowed from ancient philosophers who's names escape me right now. This method had three levels:

  • Sense Illusions: the possibility that any sense-data one might see, hear, etc does not accurately reflect reality.
  • Dream Argument: the possibility that all our experiences are fact a dream and that we have no experience or knowledge of the real world.
  • Great Deceiver: the possibility that some malicious and powerful being puts incorrect thoughts and experiences directly into our minds, for the sole purpose of deceiving us.

Eventually, Descartes decided that the only thing he could be certain of was his own existance. His logic goes: "If I think that I exist I must be either correct or deceived. If I am correct, I exist. If I am deceived, I must exist in order to be deceived; therefore, I exist.

He then went on to prove all sorts of other things, all based on his presupposition of the perfection of god. But that's another node...