Descartes' Meditations

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…all that he has come to accept as false and only then start to rebuild is foundation of knowledge. To insure the integrity of his newly acquired understanding of reality, he uses the method of doubt. It is only through this method that he can grasp the true nature of reality. After establishing the existence of himself, God, and the external world through this method, Descartes feels he now possess a clearer picture of reality.