The secrets of Nature's alchemy and the mysteries of "change" captivated both the scientific and literary imagination of the Middle Ages. Beneath the sphere of the moon in the medieval realm of mutability, the endless process of chemical transmutation of one element into another was seen as the basis of all physical change on Earth; human beings, animals, and all material things were thought to be part of a continual cycle of generation and corruption. This book investigates Chaucer's fascination with earthly mutability. Gabrovsky argues that an integrated knowledge of alchemy and physics is crucial to our understanding of the physical and psychological transformations central to Chaucer's poetry. He reveals that Chaucer's oeuvre represents a major contribution to the philosophy of physics, astronomy, alchemy, and logic, bringing to light a medieval worldview centered on the infinite variety of transformations in the natural world.