In Aristotle's physics, any motion of a heavy body was either natural (toward the earth's center) or forced (away from the center). Every forced motion was required to have an external mover. For projectiles, the medium was invoked as mover. These rules precluded a true science of dynamics. Galileo, following some earlier critics, noted that rotatory motion of homogeneous spheres would be neither natural nor forced under Aristotle's definition. Investigating such “neutral” motions, he saw that they would include the motion of a heavy body over a frictionless surface concentric with the earth. Hence, motion once imparted to a body on a horizontal plane would be perpetual, and Galileo declared that a heavy body is indifferent to motion or rest in the horizontal plane and would forever remain in whichever state it was given. This restricted principle of inertia enabled Galileo and his followers to found the science of dynamics, by which physics was immeasurably advanced, though he neglected to state explicitly the general inertial principle as formulated two years after his death by Pierre Gassendi and René Descartes.