The Index Librorum Prohibitorum was a running list of books deemed heretical, blasphemous, or otherwise morally dangerous by the Roman Catholic Church. First published in the 16th century, the Index was ostensibly a response to the Reformation, but its scope went far beyond Protestant theology. More than 4,000 titles would eventually appear on the list, including such literary classics as John Milton's Paradise Lost and Victor Hugo's The Hunchback of Notre-Dame; scientific works by Nicolaus Copernicus and Charles Darwin; even The History of the Decline and Fall of the Roman Empire.
Political philosophy, too, sometimes made the ignominious cut: The complete works of Thomas Hobbes were added in 1649. Besides containing explicit attacks on various teachings and practices of "the Church of Rome," his Leviathan was a challenge to the governing independence of the Holy See. By defending an absolute (civil) sovereign with power to decide even religious matters, Hobbes ran up against the Church's insistence that it alone was Christendom's spiritual authority.
