If he had only been convinced of what skillful astronomers agree upon today, namely, that the cubes of the distances of celestial bodies from the centers of their rotation stand to each other as the squares of the periods of their revolutions; and had he known that centrifugal forces stand to each other as the squares of their speeds divided by the diameter of their revolution, it would have been easy for him to correct certain parts of his physics, and to make it more perfect.