Two Suns Theory: Terrace 3, Purgatorio 16   Marco Lombardo articulates Dante's view of the Empire and Papacy as separate, autonomous institutions. Rome used to possess "two suns," he says, one showing the world's path and the other God's path; but over time these two lights have extinguished one another, and, switching metaphors, the sword and the shepherd's staff are now joined, much to the detriment of humanity (16.106-11). Dante's model of "two suns," each deriving its authority directly from God, challenges the medieval Christian notion of the pope as "sun" and the emperor as "moon" (based on Genesis 1:16), with the lesser sphere wholly dependent on the greater sphere for its authority and influence. Dante later writes a treatise dealing specifically with this issue of spiritual and political power: he argues in Monarchia that even the sun-moon analogy fails to prove papal dominion over temporal matters because the two spheres possess their own powers, including (Dante believed) their own light (3.16). Although he concedes that the emperor must show reverence to the pope, like a son to a father, Dante believes strongly in their independence as divinely sanctioned guides for humanity: "one is the Supreme Pontiff, to lead humankind to eternal life, according to the things revealed to us; and the other is the Emperor, to guide humankind to happiness in this world, in accordance with the teaching of philosophy" (Monarchia 3.16). A measure of the daring (and risk) in Dante's political philosophy is readily seen from a comparison of his ideas with sentiments expressed by Pope Boniface VIII in a papal bull of 1302 ("Unam Sanctam"). Adopting the common metaphor of "two swords," one each for spiritual and temporal authority, Boniface declares that they both "are in the power of the Church" and "one sword ought to be under the other and the temporal authority subject to the spiritual power." He continues by proclaiming a sort of papal infallibility, a highly ironic notion in light of Dante's treatment of the papacy, particularly under Boniface, in the Divine Comedy: "Therefore, if the earthly power errs, it shall be judged by the spiritual power, if a lesser spiritual power errs it shall be judged by its superior, but if the supreme spiritual power errs it can be judged only by God not by man." Later Church leaders evidently felt much as Boniface did, for they condemned Dante's contrary ideas as heretical and repeatedly censored his Monarchia: in 1329 a prominent cardinal ordered all copies of the work to be burned, and in the sixteenth century the book was included in the Church's Index of banned books. It wasn't until 1881 that Dante's book was removed from the list.