ALL objects attract each other with a force of gravitational attraction. He was merely defining it mathematically. Later Albert Einstein would make some improvements on this theory in his theory of relativity.
The law of universal gravitation.
If the two masses are m1 and m2 and the distance between them is r the magnitude of the force F is. The law of universal gravitation states that gravitational force between masses decreases with the distance between them according to an inverse-square law. Newtons law of gravitation statement that any particle of matter in the universe attracts any other with a force varying directly as the product of the masses and inversely as the square of the distance between them. Newtons law of universal gravitation published in 1687 states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.