If any theologian labors concerning the matters relating to the ordering of society, he wastes himself and does the most serious injury to the God who calls him, to the church for whose sake he has been called, and to her calling, by being a busybody and meddling in others’ business, which is insatiable ambition….Let them pay close attention, I pray…to the limits of their own vocation. For there are those whose vocation is the society of human beings, which magistrates rule, and there are those whose vocation is the communion of the saints, which the servants of God shepherd as leaders, as has been most rightly instituted by God.
The magistrates construct general the specific conclusions from the natural principles in the political science and appoint individual determinations adapted to human society and order, according to the reason of eternal law that has been sketched in the nature of a human being. Theologians and servants of God build general and specific conclusions upon the natural principles in the divinely inspired science, and abstaining from individual determinations accommodated to human society and order (which are a different kind of approach and investigation), they cultivate the communion of saints, and the conscience of whoever is in this communion, by spiritual determinations according to the rationale of the eternal law in the word of God and informed by Holy Scripture.
For the goal that has been set forth for the magistrate is that he ought to look after human society and the common good with respect to a person in their earthly and temporal affairs. However, the goal set forth for a theologian is that he ought to care for the society of the pious, which we have called the communion of saints, and for their salvation, in heavenly and eternal matters pertaining to God.
(The Mosaic Polity, 20-25)
For as close as the relationship is of ethics with theology, and of physics with medicine, so close—indeed I should say even closer—is the relationship of politics with jurisprudence. Where the moralist leaves off, there the theologian begins; where the physicist ends, the physician begins; and where the political scientist ceases, the jurist begins. For reasons of homogeneity, we must not leap readily across boundaries and limits, carrying from cognate arts what is only peripheral to our own. Prudence and an acute and penetrating judgment are indeed required to distinguish among similar things in these arts. It is necessary to keep constantly in view the natural and true goal and form of each art, and to attend most carefully to them, that we not exceed the limits justice lays down for each art and thereby reap another’s harvest. We should make sure that we render to each science its due (suum cuique) and not claim for our own what is alien to it.
So I have concluded that where the political scientist ceases, there the jurist begins, just as where the moralist stops the theologian begins, and  where the physicist ends the physician begins. No one denies, however, that all arts are united in practice.