Galileo's celebrated advocacy of the Copernican heliocentric theory over the Ptolemaic system of the universe. Galileo had received permission from the new Pope, Urban VII, to discuss Copernican theory in a book. Although the device of the dialogue, in which there was one spokesman for Copernicus, one for Ptolemy and one for Aristotle, allowed him to maintain a technically neutral position, its publication led to his trial before the Inquisition and sentence to perpetual house arrest. The book remained on the Index Librorum Prohibitorum until 1823.