 This development placed the subject of reference frames in a new theoretical context. Having set aside the reference frame of common sense—the frame in which the earth is at rest in the center, with the heavens revolving around it—the mechanical physics of this time naturally tied this subject with novel theoretical conceptions of motion, and its physical causes and effects. Copernicus argued for a heliocentric system, not from a physical theory of motion, but from the comparative simplicity and reasonableness that it introduced into astronomy; he worked within the established theory of the causes of celestial motions, namely, the revolutions of the heavenly spheres. After Copernicus, however—more precisely, after the model of revolving spheres was largely abandoned—determining the right frame of reference was connected with discovering the true physical causes of the planets’ motions. Philosophers such as Kepler, Descartes, Huygens, Leibniz, and Newton held vastly differing views of physical causation, motion. and the relativity of motion. They agreed, however, that the heliocentric picture was uniquely suited for giving a causal account of planetary motions, as the effects of physical actions originating in the sun. To Kepler and Descartes, for example, the rotation of the sun on its axis, in the same sense in which the planets revolved, identified it as the cause of those revolutions.