Galileo has left us with a large corpus of notes for unpublished treatises, among which is one manuscript that is particularly well-known among scholars and generally designated as De Motu. This manuscript represents, however, merely one example of numerous treatises which he had planned to write in the course of his career but actually never published or even wrote. In the case of De Motu, it is usually assumed that he decided against its publication because he realized that the physical theory expounded in this treatise is untenable. Instead, according to a widely held opinion, he embarked on a radically new pathway that eventually led him to the new physics of his famous published works, the Dialogue and the Discorsi. From this perspective, an unpublished treatise such as De Motu may appear to be relevant only to the understanding of the individual prehistory of Galileo's great scientific achievements, while the understanding of the knowledge he shared with others must be primarily based on his published writings. Our paper will focus on Galileo's unpublished writings of which De Motu is an example. We argue that, contrary to what one might expect, the preserved traces of these unpublished writings contain important clues for the reconstruction of the shared bodies and images of knowledge which essentially shaped the research of Galileo and his contemporaries and which therefore constitute the primary subject of a historical epistemology of early modern science.