Introduction

The Principia is generally regarded as the greatest work in the history of science. From definitions and three laws of motion, inertia, the relation of force and acceleration, and action and reaction, Newton derives with mathematical rigour the elliptical orbits of the planets, the tides, the paths of comets and the shape of the earth, and shows that the force that makes an apple fall is the same force that keeps the moon in its orbit: universal gravitation. For the first time the motions of the heavens and the motions on earth are brought under one set of laws. The three books consist mostly of difficult geometrical proofs. This volume translates the parts accessible to the general reader: the definitions, the laws of motion, the 'rules of reasoning in philosophy', the essentials of Book III, The System of the World, and the famous General Scholium at the end, where Newton writes: 'I frame no hypotheses.'