PHLB20H3 Lecture Notes - Lecture 7: Cogito Ergo Sum, A Priori And A Posteriori, Cartesian Doubt
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Thus, the meditator concludes, though he can doubt composite things, he cannot doubt the simple and universal parts from which they are constructed like shape, quantity, size, time, etc. While we can doubt studies based on composite things, like medicine, astronomy, or physics, he concludes that we cannot doubt studies based on simple things, like arithmetic and geometry. On further reflection, the meditator realizes that even simple things can be doubted. Omnipotent god could make even our conception of mathematics false. One might argue that god is supremely good and would not lead him to believe falsely all these things. But by this reasoning we should think that god would not deceive him with regard to anything, and yet this is clearly not true. God, then there is even greater likelihood of being deceived, since our imperfect senses would not have been created by a perfect being.
