I answer that, This is a division of penance as a virtue. Now it must be observed that every virtue acts in accordance with the time being, as also in keeping with other due circumstances, wherefore the virtue of penance has its act at this time, according to the requirements of the New Law.
Now it belongs to penance to detest one's past sins, and to purpose, at the same time, to change one's life for the better, which is the end, so to speak, of penance. And since moral matters take their species from the end, as stated in the FS, Question [1], Article [3]; FS, 18, Articles [4],6, it is reasonable to distinguish various species of penance, according to the various changes intended by the penitent.
Accordingly there is a threefold change intended by the penitent. The first is by regeneration unto a new life, and this belongs to that penance which precedes Baptism. The second is by reforming one's past life after it has been already destroyed, and this belongs to penance for mortal sins committed after Baptism. The third is by changing to a more perfect operation of life, and this belongs to penance for venial sins, which are remitted through a fervent act of charity, as stated above (Question [87], Articles [2],3).