All bodies on the Earth's surface are attracted by the Earth with a force equal to their mass multiplied by gravitational acceleration. In classical and medieval thought, gravity was regarded as an internal property of bodies that impelled them naturally toward the center of the world. Galileo (1564-1642) largely subscribed to this view. René Descartes (1596-1650) was the first to see gravity as the effect of a force exerted on bodies from the outside (the pressure exerted by "subtle matter" from the periphery toward the center of "vortexes"). He excluded gravity from the list of the properties of bodies. Isaac Newton (1642-1727), in Principia Mathematica (1687), completed the clarification of the actual nature of gravity. He established that gravity is a universal force acting on all bodies in inverse proportion to the square of distance.