 Contrary to Tycho and Kepler, Ursus held a fictionalist position in astronomy. Yet in the very beginning of his work De hypothesibus, Ursus makes a clear declaration about the nature of astronomical theories, which is very similar to the approach suggested by Osiander in his forward to Copernicus’ De revolutionibus: a hypothesis is a “fictitious supposition”, introduced just for the sake of “saving the motions of the heavenly bodies” and to “calculate them” (trans. Jardine 1988, p. 41)