Kepler's laws of planetary motion planets. However, Newton states in his Principia that he considers forces from a mathematical point of view, not a physical, thereby taking an instrumentalist view. Moreover, he does not assign a cause to gravity. Newton defined the force acting on a planet to be the product of its mass and the acceleration (see Newton's laws of motion). So: The Sun plays an unsymmetrical part, which is unjustified. So he assumed, in Newton's law of universal gravitation: As the planets have small masses compared to that of the Sun, the orbits conform approximately to Kepler's laws. Newton's model improves upon Kepler's
Three Laws of Robotics inevitable result of the "Three Laws of Humanics". This story does not fit within the overall sweep of the "Robot" and "Foundation" series; if the George robots "did" take over Earth some time after the story closes, the later stories would be either redundant or impossible. Contradictions of this sort among Asimov's fiction works have led scholars to regard the "Robot" stories as more like "the Scandinavian sagas or the Greek legends" than a unified whole. Indeed, Asimov describes "—That Thou Art Mindful of Him" and "Bicentennial Man" as two opposite, parallel futures for robots that obviate the Three Laws