Was Laplace actually a cosmologist? There is no easy answer to this question. Laplace was certainly not a cosmologist in the sense in which we can say that Aristotle or Lambert were; nor in the sense of Hubble or Bondi—for the sole reason that he never proposed any definite theory or model, or even any conjecture, concerning the Universe as a whole. Yet, in a broader but still legitimate sense, he was a cosmologist: He believed in a philosophy of nature of which one of the main tenets is that the laws of mechanics and the law of gravitation are absolutely and rigorously true throughout the Universe. He thought he had proved beyond any doubt that all the observed motions in the solar system can be explained by these laws—and thus he may be defined a cosmologist.