Rene Descartes is the philosophical architect of our modern world. In metaphysics, he established the view that mind and body are distinct substances, a position foundational for any belief that the human soul is immortal. In mathematics, he invented analytic geometry - the basis of calculus - which makes physics as we know it possible. Descartes perfected the method of proposing and testing hypotheses with experiments that anyone can repeat, which forms the basis of modern science. In optics, he discovered and described laws of refraction and reflection. In medicine, he was a pioneer in vivisection and anatomical description for understanding the human body. In physiology, his analysis of the relations among the sense organs, nerves, and the brain is still taught today. In psychology, he discovered conditioned reflexes and investigated the role of the emotions in human behavior. Descartes said there was no point in trying to refute Aristotelian Scholasticism; rather, he would simply show a better way. Some 350 years after his death, our twenty-first-century world - from mind-body dualism to heart pumps, from pop psychology to personal computers - is thoroughly Cartesian. Nothing in the modern world would alarm or surprise him were he alive today.