Objection 1. It seems that ingratitude is not a special sin. For whoever sins acts against God his sovereign benefactor. But this pertains to ingratitude. Therefore ingratitude is not a special sin.
Objection 2. Further, no special sin is contained under different kinds of sin. But one can be ungrateful by committing different kinds of sin, for instance by calumny, theft, or something similar committed against a benefactor. Therefore ingratitude is not a special sin.
Objection 3. Further, Seneca writes (De Benef. ii.): "It is ungrateful to take no notice of a kindness, it is ungrateful not to repay one, but it is the height of ingratitude to forget it." Now these do not seem to belong to the same species of sin. Therefore ingratitude is not a special sin.
On the contrary, Ingratitude is opposed to gratitude or thankfulness, which is a special virtue. Therefore it is a special sin.
I answer that, Every vice is denominated from a deficiency of virtue, because deficiency is more opposed to virtue: thus illiberality is more opposed to liberality than prodigality is. Now a vice may be opposed to the virtue of gratitude by way of excess, for instance if one were to show gratitude for things for which gratitude is not due, or sooner than it is due, as stated above (Question 106, Article 4). But still more opposed to gratitude is the vice denoting deficiency of gratitude, because the virtue of gratitude, as stated above (Question 106, Article 6), inclines to return something more. Wherefore ingratitude is properly denominated from being a deficiency of gratitude. Now every deficiency or privation takes its species from the opposite habit: for blindness and deafness differ according to the difference of sight and hearing. Therefore just as gratitude or thankfulness is one special virtue, so also is ingratitude one special sin.
It has, however, various degrees corresponding in their order to the things required for gratitude. The first of these is to recognize the favor received, the second to express one's appreciation and thanks, and the third to repay the favor at a suitable place and time according to one's means. And since what is last in the order of generation is first in the order of destruction, it follows that the first degree of ingratitude is when a man fails to repay a favor, the second when he declines to notice or indicate that he has received a favor, while the third and supreme degree is when a man fails to recognize the reception of a favor, whether by forgetting it or in any other way. Moreover, since opposite affirmation includes negation, it follows that it belongs to the first degree of ingratitude to return evil for good, to the second to find fault with a favor received, and to the third to esteem kindness as though it were unkindness.
Reply to Objection 1. In every sin there is material ingratitude to God, inasmuch as a man does something that may pertain to ingratitude. But formal ingratitude is when a favor is actually contemned, and this is a special sin.
Reply to Objection 2. Nothing hinders the formal aspect of some special sin from being found materially in several kinds of sin, and in this way the aspect of ingratitude is to be found in many kinds of sin.
Reply to Objection 3. These three are not different species but different degrees of one special sin.
Source: Summa Theologica (New Advent)