DE MONARCHIA OF DANTE ALIGHIERI
CHAPTER VIII: Argument from the prerogative of the keys consigned to Peter.
1. From the same gospel they quote the saying of Christ to Peter, "Whatsoever thou shalt loose on earth shall be loosed in heaven,"  and understand this saying to refer alike to all the Apostles, according to the text of Matthew and John.  They reason from this that the successor of Peter has been granted of God power to bind and loose all things, and then infer that he has power to loose the laws and decrees of the Empire, and to bind the laws and decrees of the temporal kingdom. Were this true, their inference would be correct.
2. But we must reply to it by making a distinction against the major premise of the syllogism which they employ. Their syllogism is this: Peter had power to bind and loose all things; the successor of Peter has like power with him; therefore the successor of Peter has power to loose and bind all things. From this they infer that he has power to loose and bind the laws and decrees of the Empire.
3. I concede the minor premise, but the major only with distinction. Wherefore I say that "all," the symbol of the universal, which is implied in "whatsoever," is never distributed beyond the scope of the distributed term. When I say, "All animals run," the distribution of "all" comprehends whatever comes under the genus "animal." But when I say, "All men run," the symbol of the universal only refers to whatever comes under the term "man." And when I say, "All grammarians run," the distribution is narrowed still further.
4. Therefore we must always determine what it is over which the symbol of the universal is distributed; then, from the recognized nature and scope of the distributed term, will be easily apparent the extent of the distribution. Now, were "whatsoever" to be understood absolutely when it is said, "Whatsoever thou shalt bind," he would certainly have the power they claim; nay, he would have even greater power, he would be able to loose a wife from her husband, and, while the man still lived, bind her to another -- a thing he can in no wise do. He would be able to absolve me, while impenitent -- a thing which God himself cannot do. 
5. So it is evident that the distribution of the term under discussion is to be taken, not absolutely, but relatively to something else. A consideration of the concession to which the distribution is subjoined will make manifest this related something. Christ said to Peter, "I will give unto thee the keys of the kingdom of heaven;" that is, I will make thee doorkeeper of the kingdom of heaven. Then he adds, "and whatsoever," that is, "everything which," and He means thereby, "Everything which pertains to that office thou shalt have power to bind and loose." And thus the symbol of the universal which is implied in "whatsoever" is limited in its distribution to the prerogative of the keys of the kingdom of heaven. Understood thus, the proposition is true, but understood absolutely, it is obviously not. Therefore I conclude that although the successor of Peter has authority to bind and loose in accordance with the requirements of the prerogative granted to Peter, it does not follow, as they claim, that he has authority to bind and loose the decrees or statutes of Empire, unless they prove that this also belongs to the office of the keys. But we shall demonstrate farther on that the contrary is true.
1. Matt. 16. 19: "And I will give unto thee the keys of the kingdom of heaven; and whatsoever thou shalt bind on earth shall be bound in heaven, and whatsoever thou shalt loose on earth shall be loosed in heaven."
This argument from the keys of Peter is set forth by Thomas Aquinas, De Regimine Principium 3. 10; S. T. 3, Supple Q. 17; in the Decretals of Gregory IX 1. 33. 6; 2. 1. 13. Dante's reverence for the pontifical office can never be questioned even in the expression of sentiments the most Ghibelline. De Mon. 2. 3.4, and note 8. The main references in the Comedy that show his attitude toward the successors of Peter are: --
Inf. 19. 90-101: "My reverence for the supreme keys ..."
Inf. 27. 103: "I have power to lock and unlock Heaven as thou knowest; since two are the keys which my forerunner held not dear." These are words put into the mouth of Boniface VIII.
The two keys which Statius held from Peter in his office as guardian of the gate of Purgatory are described Purg. 9. 117 ff.
Par. 23. 136: "Here triumphs, under the high Son of God, and of Mary, for his victory,... he who holds the keys of such glory."
Par. 24. 34: "O eternal light of the great man to whom our Lord left the keys, which He bore below, of this wondrous joy."
Par. 27. 46: "It was not our intention ... that the keys which were granted to me should become a device on a banner to fight against men baptized." So Peter rebukes the wickedness of the Church and its officials.
Par. 32. 124: "On the right behold that ancient Father of Holy Church to whom Christ entrusted the keys of this lovely flower."
2. Matt. 18. 18; John 20. 23.
3. Rom. 7. 3. Inf. 27. 118: "Absolved he cannot be, who does not repent; nor is it possible to repent and to will at the same time, by reason of the contradiction which agrees not in it."