Galileo meanwhile had retired to Florence and devoted himself to mechanical science, (of which his work is the foundation) though constantly harassed by much ill health and many family perplexities. At the advice of his friends, he allowed the attacks on the Copernican doctrine to go unanswered,[257] till with the accession to the papacy in 1623 of Cardinal Barberini, as Urban VIII, a warm admirer and supporter of his, he thought relief was in sight. He was further cheered by a conversation Cardinal di Zollern reported having had with Pope Urban, in which his Holiness had reminded the Cardinal how he (the Pope) had defended Copernicus in the time of Paul V, and asserted that out of just respect owed to the memory of Copernicus, if he had been pope then, he would not have permitted his opinion to be declared heretical.[258] Feeling that he now had friends in power, Galileo began his great work, _Dialogo sopra i Due Sistemi Massimi del Mondo_, a dialogue in four "days" in which three interlocutors discuss the arguments for and against the Copernican theory, though coming to no definite conclusion. Sagredo was an avowed Copernican and Galileo's spokesman, Salviati was openminded, and the peripatetic was Simplicio, appropriately named for the famous Sicilian sixth century commentator on Aristotle.[259] [Footnote 257: In 1620 the Congregation issued the changes it required to have made in the _De Revolutionibus_. They are nine in all, and consist mainly in changing assertion of the earth's movement to hypothetical statement and in striking out a reference to the earth as a planet. Doc. in Favaro: 140-141. See illustration, p. 61.] [Footnote 258: Doc. in Favaro: 149.] [Footnote 259: Galileo: _Dialogo_: To the Reader.] [Illustration: A "CORRECTED" PAGE FROM THE _De Revolutionibus_. A photographic facsimile (reduced) of a page from Mulier's edition (1617) of the _De Revolutionibus_ as "corrected" according to the _Monitum_ of the Congregations in 1620. The first writer underlined the passages to be deleted or altered with marginal notes indicating the changes ordered; the second writer scratched out these passages, and wrote out in full the changes the other had given in abbreviated form. The _Notæ_ are Mulier's own, and so were not affected by the order. The effect of the page is therefore somewhat contradictory!]