In Meditations on First Philosophy, René Descartes uses rational thought to argue that his essence exists in his thoughts alone. He reasons that the mind and body are distinct and separable, and that the mind, as the originator of thought, is what defines the person. This guide will outline the reasoning Descartes' uses in establishing his theory of knowledge that he argues is necessary to form a 'proper foundation' for the sciences and move it away from the medieval systems that were the prevailing orthodoxies of the time.