Sir Isaac Newton is famous for his theory of gravity, which fundamentally changed our understanding of physics. His law of universal gravitation, articulated in his work "Philosophiæ Naturalis Principia Mathematica," asserts that every mass attracts every other mass in the universe with a force proportional to their masses and inversely proportional to the square of the distance between their centers. This theory provided a cohesive framework for understanding celestial and terrestrial motion, explaining how planets orbit the sun and objects fall to the ground. Newton's work laid the groundwork for classical mechanics and has had a lasting impact on science.