Despite his demanding religious responsibilities, Paolo Sarpi maintained an active involvement in science between 1578 and 1598 – as hisPensierireveal. They show that from 1585 onwards he studied the Copernican theory and recorded arguments in its favour. The fact that for 1595 they include an outline of a Copernican tidal theory resembling Galileo'sDialoguetheory is well known. But examined closely, Sarpi's theory is found to be different from that of theDialoguein several important respects. That Sarpi was a Copernican by 1592 is (...) revealed by other of hispensieri, whereas at that time we know that Galileo was not. The examination of Sarpi's tidal theory and of the work of Galileo in this period indicates that the theory Sarpi recorded in 1595 was of his own creation. The appreciation that the theory was Sarpi's and that Galileo subsequently came to change his views on the Copernican theory and adopted the tidal theory has major implications for our understanding of the significance of Sarpi's contribution to the Scientific Revolution. Moreover, it appears that several of the most significant theoretical features of the tidal theory published by Galileo in theDialogue –and which proved of lasting value – were in reality Sarpi's. (shrink)