Objection 1. It would seem that these three are not integral parts of Penance. For, as stated above (Question 84, Article 3), Penance is ordained against sin. But sins of thought, word, and deed are the subjective and not integral parts of sin, because sin is predicated of each one of them. Therefore in Penance also, contrition in thought, confession in word, and satisfaction in deed are not integral parts.
Objection 2. Further, no integral part includes within itself another that is condivided with it. But contrition includes both confession and satisfaction in the purpose of amendment. Therefore they are not integral parts.
Objection 3. Further, a whole is composed of its integral parts, taken at the same time and equally, just as a line is made up of its parts. But such is not the case here. Therefore these are not integral parts of Penance.
On the contrary, Integral parts are those by which the perfection of the whole is integrated. But the perfection of Penance is integrated by these three. Therefore they are integral parts of Penance.
I answer that, Some have said that these three are subjective parts of Penance. But this is impossible, because the entire power of the whole is present in each subjective part at the same time and equally, just as the entire power of an animal, as such, is assured to each animal species, all of which species divide the animal genus at the same time and equally: which does not apply to the point in question. Wherefore others have said that these are potential parts: yet neither can this be true, since the whole is present, as to the entire essence, in each potential part, just as the entire essence of the soul is present in each of its powers: which does not apply to the case in point. Therefore it follows that these three are integral parts of Penance, the nature of which is that the whole is not present in each of the parts, either as to its entire power, or as to its entire essence, but that it is present to all of them together at the same time.
Reply to Objection 1. Sin forasmuch as it is an evil, can be completed in one single point, as stated above (2, ad 4); and so the sin which is completed in thought alone, is a special kind of sin. Another species is the sin that is completed in thought and word: and yet a third species is the sin that is completed in thought, word, and deed; and the quasi-integral parts of this last sin, are that which is in thought, that which is in word, and that which is in deed. Wherefore these three are the integral parts of Penance, which is completed in them.
Reply to Objection 2. One integral part can include the whole, though not as to its essence: because the foundation, in a way, contains virtually the whole building. In this way contrition includes virtually the whole of Penance.
Reply to Objection 3. All integral parts have a certain relation of order to one another: but some are only related as to position, whether in sequence as the parts of an army, or by contact, as the parts of a heap, or by being fitted together, as the parts of a house, or by continuation, as the parts of a line; while some are related, in addition, as to power, as the parts of an animal, the first of which is the heart, the others in a certain order being dependent on one another: and thirdly some are related in the order of time: as the parts of time and movement. Accordingly the parts of Penance are related to one another in the order of power and time, since they are actions, but not in the order of position, since they do not occupy a place.
Source: Summa Theologica (New Advent)