• Like Aristotle, Descartes believes it necessary to build a structure of knowledge on a sure foundation...
• ...But he considers contemporary knowledge to be suspect: we know that ideas change - how can be have any faith that the scientific explanations we hold to today will not be overturned tomorrow? (Galileo just before Descartes)
• Resolves to establish a foundation and structure based only on facts of which he can be certain
• Criterion for something being certain is that it cannot be doubted
• To doubt X does not mean to regard X as false: rather, he uses doubt as a tool that can be used to eliminate anything which is not 100% certain
• Descartes considers various aspects of reality to decide whether they can be doubted:
  • immediate experience through the senses is no good, since senses can deceive me: they're not 100% trustworthy
  • but even I can't be sure my senses give a true picture of it, or if I'm dreaming, then at least the experience or dreams I have must be inspired by something real: things like shapes, size, numbers must be real?
  • but the ideas of things like size, shape, and even the laws of mathematics could (so far as I know) be being corrupted in me by God
  • I can't even be sure of the existence of God
  All of these are subject to doubt, especially if one considers the idea of a malicious demon
• But for me to doubt means that someone is doubting: even if the demon is deceiving me, then I exist because I am being deceived
• therefore I at least know that so long as I continue to think, I must exist

Problems

• Perhaps our minds are fundamentally incapable of grasping true knowledge.
• Descartes doesn't appear to doubt his powers of reason. But we have evidence that these can be suspect. E.g. quote from Solomon book p129

  You don't think in depression that you've put on a grey veil and are seeing the world through the haze of a bad mood. You think that the veil has been taken away, the veil of happiness, and now you're seeing truly. You try to pin the truth down and take it apart, and you think that the truth is a fixed thing, but the truth is alive and it runs around...it's much harder with depressed people because we believe we are seeing the truth. But the truth lies."

• The "foundational approach" may not be possible - e.g. when computers are designed nowadays, we use computers to do it. Similarly, Descartes can't throw away his intellect and reasoning, and so perhaps his argument is not free of whatever corrupting influence this may bring.

  "We are part of the very universe we are trying to understand. We are trapped inside the system. There is no Archimedean point on which to stand to behold all of creation"