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René Descartes (1596–1650)

Descartes was the first of the modern rationalists and has been dubbed the 'Father of Modern Philosophy.' Much subsequent Western philosophy is a response to his writings, which are studied closely to this day.

Descartes thought that only knowledge of eternal truths – including the truths of mathematics, and the epistemological and metaphysical foundations of the sciences – could be attained by reason alone; other knowledge, the knowledge of physics, required experience of the world, aided by the scientific method. He also argued that although dreams appear as real as sense experience, these dreams cannot provide persons with knowledge. Also, since conscious sense experience can be the cause of illusions, then sense experience itself can be doubtable. As a result, Descartes deduced that a rational pursuit of truth should doubt every belief about sensory reality. He elaborated these beliefs in such works as Discourse on the Method, Meditations on First Philosophy, and Principles of Philosophy. Descartes developed a method to attain truths according to which nothing that cannot be recognised by the intellect (or reason) can be classified as knowledge. These truths are gained "without any sensory experience," according to Descartes. Truths that are attained by reason are broken down into elements that intuition can grasp, which, through a purely deductive process, will result in clear truths about reality.

Descartes therefore argued, as a result of his method, that reason alone determined knowledge, and that this could be done independently of the senses. For instance, his famous dictum, cogito ergo sum or "I think, therefore I am", is a conclusion reached a priori i.e., prior to any kind of experience on the matter. The simple meaning is that doubting one's existence, in and of itself, proves that an "I" exists to do the thinking. In other words, doubting one's own doubting is absurd. This was, for Descartes, an irrefutable principle upon which to ground all forms of other knowledge. Descartes posited a metaphysical dualism, distinguishing between the substances of the human body (" res extensa ") and the mind or soul (" res cogitans "). This crucial distinction would be left unresolved and lead to what is known as the mind–body problem, since the two substances in the Cartesian system are independent of each other and irreducible.