René Descartes believed he had proven the existence of God through his use of systematic doubt in his book:

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René Descartes believed he had proven the existence of God through his use of systematic doubt in his book "Discourse on the Method". In this book, Descartes argues that since he can doubt the existence of everything, except for his own thinking, he can be certain that he exists as a thinking thing. From there, he argues that the idea of God must also be innate within him and that since the idea of God includes necessary existence, God must exist. This argument is known as the ontological argument for the existence of God.

René Descartes Believed He Had Proven the Existence of God