The 1687 book in which Isaac Newton showed that the same simple mathematical laws govern a falling apple and the orbiting moon — uniting heaven and earth under one system of motion and gravity, and founding the science of physics.
In the inherited Aristotelian picture, the heavens and the earth obeyed different rules — stars and planets by one logic, falling stones by another — though Galileo and Kepler had already begun to weaken that divide. Newton's Principia collapsed that ancient divide into a single sentence of mathematics. The force that pulls an apple to the ground is the same force that holds the moon in its orbit, and both follow one law that can be written down and solved. With that move Newton turned the universe into a system of exact, predictable rules — and founded classical mechanics, the model for all of modern physics.