The first source of duty is prudence, from whence spring three other virtues; and they cannot be separated or torn asunder, since they are mutually connected one with the other.
126. The first source of duty, then, is prudence. For what is more of a duty than to give to the Creator all one's devotion and reverence? This source, however, is drawn off into other virtues. For justice cannot exist without prudence, since it demands no small amount of prudence to see whether a thing is just or unjust. A mistake on either side is very serious. “For he that says a just man is unjust, or an unjust man is just, is accursed with God. Wherefore does justice abound unto the wicked?” says Solomon. Nor, on the other hand, can prudence exist without justice, for piety towards God is the beginning of understanding. On which we notice that this is a borrowed rather than an original idea among the worldly wise, for piety is the foundation of all virtues.
127. But the piety of justice is first directed towards God; secondly, towards one's country; next, towards parents; lastly, towards all. This, too, is in accordance with the guidance of nature. From the beginning of life, when understanding first begins to be infused into us, we love life as the gift of God, we love our country and our parents; lastly, our companions, with whom we like to associate. Hence arises true love, which prefers others to self, and seeks not its own, wherein lies the pre-eminence of justice.
128. It is ingrained in all living creatures, first of all, to preserve their own safety, to guard against what is harmful, to strive for what is advantageous. They seek food and converts, whereby they may protect themselves from dangers, storms, and sun—all which is a mark of prudence. Next we find that all the different creatures are by nature wont to herd together, at first with fellows of their own class and sort, then also with others. So we see oxen delighted to be in herds, horses in droves, and especially like with like, stags, also, in company with stags and often with men. And what should I say on their desire to have young, and on their offspring, or even on their passions, wherein the likeness of justice is conspicuous?
129. It is clear, then, that these and the remaining virtues are related to one another. For courage, which in war preserves one's country from the barbarians, or at home defends the weak, or comrades from robbers, is full of justice; and to know on what plan to defend and to give help, how to make use of opportunities of time and place, is the part of prudence and moderation, and temperance itself cannot observe due measure without prudence. To know a fit opportunity, and to make return according to what is right, belongs to justice. In all these, too, large-heartedness is necessary, and fortitude of mind, and often of body, so that we may carry out what we wish.
Source: On the Duties of the Clergy (New Advent)