This is the first English translation of Newton’s masterwork in physics. The Copernican idea that the Earth moves as a planet required a thorough revision of physics. Galileo undertook this task in his Discourse on Two New Sciences, published 80 years after Copernicus. With a mathematical description of the law of universal gravitation, Newton in this book unified the terrestrial physics of Galileo with the celestial mechanics of Kepler’s laws. The development of science from Copernicus to Newton then became recognized as a “Scientific Revolution,” a complete overthrow of Aristotelian physics and cosmology.