Newton explained the inverse square law in Principia. On looking through an English translation, though, I'm having difficulty pulling out a single quote that is Newton's clearest statement of the idea that point particles attracts all point particles with a force that is inversely proportional to the square of the distance and proportional to each of the masses.
I see proofs of the shell theorem that state that the force between a point and a sphere obeys this law, and I see proofs that if we start by assuming an inverse square law, we get conic section orbits. But the book is very large and difficult to read; I simply can't find the best passage stating the universal gravitational law in Newton's own words. What is the best passage to use?