 Yet even with all these developments, Galileo still needed to work out general principles concerning the nature of motion for this newly unified matter. In this respect, Galileo differed from Ptolemy (at least of the Almagest), Copernicus, or even Tycho Brahe, who treated their planetary systems—be they Earth- or sun-centered—merely as models of the planets’ observed motions; that is, as mathematical conceits for calculating observable positions. For Galileo, by contrast, Copernicanism was also a commitment to a physically realizable cosmography. Consequently, he needed to work out, at least qualitatively, a way of thinking about the actual motions of matter. He had to devise (or shall we say, discover) principles of local motion that would fit a central sun, planets moving around that sun, a whirling Earth, and everything on it.