Objection 1. It seems that thankfulness is not a special virtue, distinct from other virtue. For we have received the greatest benefits from God, and from our parents. Now the honor which we pay to God in return belongs to the virtue of religion, and the honor with which we repay our parents belongs to the virtue of piety. Therefore thankfulness or gratitude is not distinct from the other virtues.
Objection 2. Further, proportionate repayment belongs to commutative justice, according to the Philosopher (Ethic. v, 4). Now the purpose of giving thanks is repayment (Ethic. 5,4). Therefore thanksgiving, which belongs to gratitude, is an act of justice. Therefore gratitude is not a special virtue, distinct from other virtues.
Objection 3. Further, acknowledgment of favor received is requisite for the preservation of friendship, according to the Philosopher (Ethic. viii, 13; ix, 1). Now friendship is associated with all the virtues, since they are the reason for which man is loved. Therefore thankfulness or gratitude, to which it belongs to repay favors received, is not a special virtue.
On the contrary, Tully reckons thankfulness a special part of justice (De Invent. Rhet. ii).
I answer that, As stated above (I-II, 60, 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.
Reply to Objection 1. Just as religion is superexcelling piety, so is it excelling thankfulness or gratitude: wherefore giving thanks to God was reckoned above (Question 83, Article 17) among things pertaining to religion.
Reply to Objection 2. Proportionate repayment belongs to commutative justice, when it answers to the legal due; for instance when it is contracted that so much be paid for so much. But the repayment that belongs to the virtue of thankfulness or gratitude answers to the moral debt, and is paid spontaneously. Hence thanksgiving is less thankful when compelled, as Seneca observes (De Beneficiis iii).
Reply to Objection 3. Since true friendship is based on virtue, whatever there is contrary to virtue in a friend is an obstacle to friendship, and whatever in him is virtuous is an incentive to friendship. On this way friendship is preserved by repayment of favors, although repayment of favors belongs specially to the virtue of gratitude.
Source: Summa Theologica (New Advent)