The most influential physicist who lived at this time was Isaac Newton. Newton laid the foundations for all of modern classical physics, and in 1687 he published Principia Mathematica which established the theory of universal gravitation and his laws of motion. Newton developed all of classical physics as we know it, and also devised a mathematical tool he called 'fluctions', which we now know as differential calculus. He was urged to publish his work by Edmund Halley, who in 1705 observed a comet and predicted its return.