I answer that, As stated above (FS, Question [60], Article [3]), the nature of the debt to be paid must needs vary according to various causes giving rise to the debt, yet so that the greater always includes the lesser. Now the cause of debt is found primarily and chiefly in God, in that He is the first principle of all our goods: secondarily it is found in our father, because he is the proximate principle of our begetting and upbringing: thirdly it is found in the person that excels in dignity, from whom general favors proceed; fourthly it is found in a benefactor, from whom we have received particular and private favors, on account of which we are under particular obligation to him.
Accordingly, since what we owe God, or our father, or a person excelling in dignity, is not the same as what we owe a benefactor from whom we have received some particular favor, it follows that after religion, whereby we pay God due worship, and piety, whereby we worship our parents, and observance, whereby we worship persons excelling in dignity, there is thankfulness or gratitude, whereby we give thanks to our benefactors. And it is distinct from the foregoing virtues, just as each of these is distinct from the one that precedes, as falling short thereof.