 The other aspect of all this that has been hotly debated is what constitutes proof or demonstration of a scientific claim. Galileo believed he had a proof of terrestrial motion. This argument concerning the cause of the tides is contained in On the Ebb and Flow of the Tides, a manuscript he composed in 1616 while Copernicanism was under the Inquisition’s scrutiny, and the main thrust of which appears in Day Four of the Dialogue Concerning the Two Chief World Systems.