known as the Regulæ Juris. His contribution came to be known as the Liber Sextus. This material is still of importance to canon lawyers or canonists today, to interpret and analyze the canons and other forms of ecclesiastical law properly. The "Regulae Iuris" appear at the end of the Liber Sextus (in VI°), and now published as part of the five Decretales in the Corpus Juris Canonici. They appear as simple aphorisms, such as "Regula VI: Nemo potest ad impossibile obligari." ('No one can be obligated for something impossible.') Other systems of law also have their own Regulæ Juris, whether by the same name or something serving a similar function. Cardinals Boniface VIII put forward some of the strongest claims of any pope to temporal as well as spiritual power. He involved himself often with foreign affairs. In his Papal bull of 1302, Unam sanctam, Boniface VIII stated that since the Church is one, since the Church is necessary for salvation, and since Christ appointed Peter to lead it, it is "absolutely necessary for salvation that every human creature be subject to the Roman pontiff". These views, and his chronic intervention in "temporal" affairs, led to many bitter quarrels with Albert I of Germany, Philip IV of France, and Dante Alighieri, who wrote his treatise De Monarchia to dispute Boniface's claims of papal supremacy. In 1297, Cardinal Jacopo Colonna disinherited his brothers Ottone, Matteo, and Landolfo of their lands. The latter three appealed to Pope Boniface VIII, who ordered Jacopo to return the land and furthermore to hand over the family's strongholds of Colonna, Palestrina, and other towns to the Papacy. Jacopo refused. Jacopo Colonna and his nephew, Pietro Colonna, had also seriously compromised themselves by maintaining highly questionable relations with the political enemies of the pope, James II of Aragon and Frederick III of Sicily. In May, Boniface removed them from the College of Cardinals and excommunicated them and their followers. The Colonna family (aside from the three brothers allied with the Pope) declared that Boniface had been elected illegally following the unprecedented abdication of Pope Celestine V. The dispute led to open warfare, and in September Boniface appointed Landolfo to the command of his army to put down the revolt of Landolfo's relatives. By the end of 1298 Landolfo had captured Colonna, Palestrina and other towns and razed them to the ground after they had surrendered peacefully under