Throughout his scientific career, Galileo conducted research into the nature and laws of motion. This quest culminated in the formulation of a totally new dynamics based on mathematical reasoning and ingenious experiments, which he set out in Discorsi e dimostrazioni matematiche [Mathematical discourses and demonstrations] (Leyden 1638). In this seminal book, Galileo replaced the Aristotelian concept of motion with a vision based on new principles: acceleration is a natural property that remains constant over time; the conservation and composition of motion; the parabolic path of projectiles; the infinite force of an impact; and so on. For Galileo, the radical reform of Aristotelian dynamics also paved the way for the definitive assertion of the Copernican vision.