...according to different causes why something is owed, it is necessary to vary the account of repayment of the debt, so that there is always in the greater what is contained in the less. But in God is first and forewmost found a cause of debt, in that he is first principle of all our goods. And secondarily, in the parent, who is the proximate principle of our generation and upbringing (disciplinae). And thirdly, in the person who excels in dignity, from whom common benefits proceed. And fourthly, in some benefactor from which we obtain some particular and private benefits, for which we are obligated to him particularly. Therefore because what we owe God, or parent, or person excelling in dignity, is not what we owe to some benefactor from whom we receive particular benefit, it follows that after religion, by which we pay due worship to God, and piety, by which we honor parents, and observance, by which we honor persons excelling in dignity, is thanks (gratia) or gratitude, by which benefactors are repayed with thanks. And it is distinct from the previous virtues, as what is posterior is distinguished from the prior, as if it were deficient with respect to it.
[Thomas Aquinas, Summa Theologiae, 2-2.106.1, my rough translation. The Dominican Fathers translation is here; the Latin is here. All of these are concerned with moral debts -- that is, cases in which we can be said to owe something that does not admit of full and adequate repayment. Debts that can be canceled or terminated by payment -- in which, owing you something, I can give you a benefit that is proportionally equal to what you have given me -- fall under justice in the strict sense.]
