§ III. On the Potential Parts of Justice

The potential parts of justice are several. For justice is a virtue toward another, rendering to him what is owed according to equality. Therefore every virtue which by its nature is toward another, but falls short of justice by reason either of debt or of equality, must be joined to justice as a potential part. Now the virtues which fall short by reason of debt, or in other words which regard not a juridical debt but only a moral debt, that is, one to be rendered from the moral goodness of virtue alone, are of two species. For some regard a debt "so necessary that without it moral goodness of conduct cannot be preserved" (St. Thomas, II-II, q. 80, single article); and these are truth, gratitude, and vindication. Others regard a debt which "is necessary as contributing to greater moral goodness, without which nevertheless moral goodness can be preserved" (St. Thomas, cited place); and these are liberality and affability. By reason of equality those virtues fall short, or in other words regard a debt which can in no way be rendered according to equality: religion, penance, piety, and observance.

Therefore there are nine virtues which are referred to justice as potential parts; two of them regard a debt toward God, namely religion and penance; the remaining seven have as their object a debt toward the neighbor.