Objection 1. It would seem that the ingratitude, whereby a subsequent sin causes the return of sins previously forgiven, is a special sin. For the giving of thanks belongs to counterpassion which is a necessary condition of justice, as the Philosopher shows (Ethic. v, 5). But justice is a special virtue. Therefore this ingratitude is a special sin.
Objection 2. Further, Tully says (De Inv. Rhet. ii) that thanksgiving is a special virtue. But ingratitude is opposed to thanksgiving. Therefore ingratitude is a special sin.
Objection 3. Further, a special effect proceeds from a special cause. Now ingratitude has a special effect, viz. the return, after a fashion, of sins already forgiven. Therefore ingratitude is a special sin.
On the contrary, That which is a sequel to every sin is not a special sin. Now by any mortal sin whatever, a man becomes ungrateful to God, as evidenced from what has been said (1). Therefore ingratitude is not a special sin.
I answer that, The ingratitude of the sinner is sometimes a special sin; and sometimes it is not, but a circumstance arising from all mortal sins in common committed against God. For a sin takes its species according to the sinner's intention, wherefore the Philosopher says (Ethic. v, 2) that "he who commits adultery in order to steal is a thief rather than an adulterer."
If, therefore, a sinner commits a sin in contempt of God and of the favor received from Him, that sin is drawn to the species of ingratitude, and in this way a sinner's ingratitude is a special sin. If, however, a man, while intending to commit a sin, e.g. murder or adultery, is not withheld from it on account of its implying contempt of God, his ingratitude will not be a special sin, but will be drawn to the species of the other sin, as a circumstance thereof. And, as Augustine observes (De Nat. et Grat. xxix), not every sin implies contempt of God in His commandments. Therefore it is evident that the sinner's ingratitude is sometimes a special sin, sometimes not.
This suffices for the Replies to the Objections: for the first (three) objections prove that ingratitude is in itself a special sin; while the last objection proves that ingratitude, as included in every sin, is not a special sin.
Source: Summa Theologica (New Advent)