Building on the work of Galileo and Kepler, Isaac Newton developed the first quantitative theory of gravitation, which he published in his Principa in 1687. Newton held that every particle of matter in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. Mathematically this is expressed by the classical formula F=Gm1m2/d2, where F is the force of attraction, m1 and m2 are the masses, d is the distance, and G is a universal gravitational constant whose value depends on the units chosen to express it.