Objection 1. It would seem that sins are unfittingly divided into sins of thought, word, and deed. For Augustine (De Trin. xii, 12) describes three stages of sin, of which the first is "when the carnal sense offers a bait," which is the sin of thought; the second stage is reached "when one is satisfied with the mere pleasure of thought"; and the third stage, "when consent is given to the deed." Now these three belong to the sin of thought. Therefore it is unfitting to reckon sin of thought as one kind of sin.
Objection 2. Further, Gregory (Moral. iv, 25) reckons four degrees of sin; the first of which is "a fault hidden in the heart"; the second, "when it is done openly"; the third, "when it is formed into a habit"; and the fourth, "when man goes so far as to presume on God's mercy or to give himself up to despair": where no distinction is made between sins of deed and sins of word, and two other degrees of sin are added. Therefore the first division was unfitting.
Objection 3. Further, there can be no sin of word or deed unless there precede sin of thought. Therefore these sins do not differ specifically. Therefore they should not be condivided with one another.
On the contrary, Jerome in commenting on Ezekiel 43:23: "The human race is subject to three kinds of sin, for when we sin, it is either by thought, or word, or deed."
I answer that, Things differ specifically in two ways: first, when each has the complete species; thus a horse and an ox differ specifically: secondly, when the diversity of species is derived from diversity of degree in generation or movement: thus the building is the complete generation of a house, while the laying of the foundations, and the setting up of the walls are incomplete species, as the Philosopher declares (Ethic. x, 4); and the same can apply to the generation of animals. Accordingly sins are divided into these three, viz. sins of thought, word, and deed, not as into various complete species: for the consummation of sin is in the deed, wherefore sins of deed have the complete species; but the first beginning of sin is its foundation, as it were, in the sin of thought; the second degree is the sin of word, in so far as man is ready to break out into a declaration of his thought; while the third degree consists in the consummation of the deed. Consequently these three differ in respect of the various degrees of sin. Nevertheless it is evident that these three belong to the one complete species of sin, since they proceed from the same motive. For the angry man, through desire of vengeance, is at first disturbed in thought, then he breaks out into words of abuse, and lastly he goes on to wrongful deeds; and the same applies to lust and to any other sin.
Reply to Objection 1. All sins of thought have the common note of secrecy, in respect of which they form one degree, which is, however, divided into three stages, viz. of cogitation, pleasure, and consent.
Reply to Objection 2. Sins of words and deed are both done openly, and for this reason Gregory (Moral. iv, 25) reckons them under one head: whereas Jerome (in commenting on Ezekiel 43:23) distinguishes between them, because in sins of word there is nothing but manifestation which is intended principally; while in sins of deed, it is the consummation of the inward thought which is principally intended, and the outward manifestation is by way of sequel. Habit and despair are stages following the complete species of sin, even as boyhood and youth follow the complete generation of a man.
Reply to Objection 3. Sin of thought and sin of word are not distinct from the sin of deed when they are united together with it, but when each is found by itself: even as one part of a movement is not distinct from the whole movement, when the movement is continuous, but only when there is a break in the movement.
Source: Summa Theologica (New Advent)