Descartes and Dualism “I think, therefore I am,” the well-known quote by René Descartes, is the basis of his theory known as dualism. The blending of mind and body or res extensa (extended substance) and res cogitans (thinking substance) reflects Descartes' ideas of a “true human being” (Cottingham 7). Known as the father of modern philosophy, Descartes realized that you could not analyze a problem simply at the level of common sense, but that you had to “probe down to the micro level” (Cottingham 4). Through his technique of doubting everything he believed to exist and establishing a new philosophy, Descartes discovered that without doubt, the only thing he could truly believe existed was his own mind. He then assumed that a demon was tricking him into believing what he saw. With this idea, he concluded that “all external things are only the illusions of dreams” (Meditations of Descartes cited in Cottingham 23) that the demon invented. By being able to convince oneself of ideas and by being able to let oneself be deceived by...