Principles of Philosophy was written by René Descartes and published in 1644. Descartes intended it to be his magnum opus—the synthesis of all his theories in physics and philosophy divided into four Parts. Part I is the only part of Principles of Philosophy that we, today, would call “philosophy.” It is an account of Descartes' epistemology and his metaphysics. The rest of the work deals with Descartes’s natural philosophy, or what we would call “science.”

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