Policraticus was the first book of political science to be produced during the Middle Ages. Written by John of Salisbury around 1159, it belongs to the genre of advice literature known as "mirrors for princes", and is addressed to the territorial ruler. Though it takes up a wide variety of ethical questions, it is most famous for attempting to define the responsibilities of kings and their relationship to their subjects.
Policraticus is a pseudo Greco-Roman neologism term, meaning "the statesman." Salisbury drew his arguments from several different sources, including the Bible and the Justinian Code. He argued for the divine right of kings, saying that
...the prince stands on a pinnacle which is exalted and made splendid with all the great and high privileges which he deems necessary for himself. And rightly so, because nothing is more advantageous to the people than that the needs of the prince should be fully satisfied; since it is impossible that his will should be found opposed to justice. Therefore, according to the usual definition, the prince is the public power, and a kind of likeness on earth of the divine majesty. Beyond doubt a large share of the divine power is shown to be in princes by the fact that at their nod men bow their necks and for the most part offer up their heads to the axe to be struck off, and, as by a divine impulse, the prince is feared by each of those over whom he is set as an object of fear. And this I do not think could be, except as a result of the will of God. For all power is from the Lord God, and has been with Him always, and is from everlasting.
This divine source allowed princes to wage just wars and required all subjects to submit to the prince's will. Salisbury argued that the prince had four responsibilities: Revere God, adore his subjects, exert self-discipline and instruct his ministers. Salisbury advocated strict punishments for lèse majesté, but argued that a prince should err on the side of mercy and compassion when enforcing his laws.
While recognizing a prince's supreme temporal power, Salisbury argued that princes must be subordinate to the will of God and the Church.
For myself, I am satisfied and persuaded that loyal shoulders should uphold the power of the ruler; and not only do I submit to his power patiently, but with pleasure, so long as it is exercised in subjection to God and follows His ordinances. But on the other hand if it resists and opposes the divine commandments, and wishes to make me share in its war against God; then with unrestrained voice I answer back that God must be preferred before any man on earth. Therefore inferiors should cleave and cohere to their superiors, and all the limbs should be in subjection to the head; but always and only on condition that religion is kept inviolate.
In Salisbury's mind, a tyrant sets a poor example for his people and could lead them from God. His example was the Roman Emperor Julian the Apostate, who attempted to restore Rome's pagan religion. In this case, Salisbury argued that killing a regent, when all other resources were exhausted, was not only justifiable but necessary, and he called a tyrant an "image of depravity . . . [who] spring(s) from evil and should be cut down with the axe wherever he grows." This may be the first defense of tyrannicide to be written after Antiquity.