Principia

Principia most commonly refers to Philosophiae Naturalis Principia Mathematica, Isaac Newton’s 1687 treatise on motion and the law of gravitation. Written in Latin and published with the help of the Royal Society, it is considered one of the most influential works in the history of science, establishing the framework of classical mechanics and celestial dynamics.

The work is organized into three books. Book I develops the fundamental principles of motion, introducing what would later be known as Newton’s three laws of motion and the concept of force as a cause of acceleration. It provides geometric demonstrations showing how forces acting on bodies produce predictable motions, including the derivation of planetary orbits from a central inverse-square force. Book II extends these ideas to the motion of bodies in resisting media and to systems where forces may vary. Book III applies the results to the entire system of the world, tying together the motions of the planets, the Moon, comets, and Earth under the universal law of gravitation.

The Principia’s core achievement is the universal gravitation law, which posits that every two bodies attract each other with a force proportional to their masses and inversely proportional to the square of the distance between them. This framework explained a wide range of phenomena—from tides and planetary orbits to the perturbations observed in comet paths—and laid the groundwork for classical physics and astronomy for centuries. Newton published subsequent editions (notably in 1713 and 1726) that enriched proofs and clarified arguments.

The term Principia can also refer to other works bearing the title, such as Principia Mathematica by Russell and Whitehead (1910–1913), but the 1687 Principia is the best known.