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Descartes realized by the very fact that he could doubt, even the existence of God, meant that he could think. Thinking must then be the first principle of philosophy and the only proof of human existence: I think therefore I am. Descartes made reason and the exploration of the mind the focus and foundation of his philosophy. His separation of matter from thinking led to the separation of the mind from the body. This led to the emergence of a separate, thinking self, a self that can create a unique mental experience of sensations. Rationalism was born and would provide a powerful and viable alternative to empiricism.