Galileo's 1632 Dialogue on the Two Chief World Systems synthesized his arguments for the Copernican system. Pope Urban VIII made a condition of its publication that Galileo should also present the views of Ptolemaic and Peripatetic (Aristotelean) astronomy. The frontispiece depicts Aristotle, Ptolemy, and Copernicus in conversation. Note behind them, however, are fortifications, guns, and a galley. Galileo's spokesman for the Copernican system (Salviati) would refer to these in explaining his new philosophy of nature.