I answer that, In every series of things moving and moved ordained to one another, we find that the inferior mover has its proper movement, and besides this, it follows, in some respect, the movement of the superior mover: this is seen in the movement of the planets, which, in addition to their proper movements, follow the movement of the first heaven. Now, in all the moral virtues, the first mover is prudence, which is called the charioteer of the virtues. Consequently each moral virtue, in addition to its proper movement, has something of the movement of prudence: and therefore, since penance is a moral virtue, as it is a part of justice, in addition to its own act, it acquires the movement of prudence. Now its proper movement is towards its proper object, which is a sin committed. Wherefore its proper and principal act, viz. contrition, essentially regards past sins alone; but, inasmuch as it acquires something of the act of prudence, it regards future sins indirectly, although it is not essentially moved towards those future sins. For this reason, he that is contrite, is sorry for his past sins, and is cautious of future sins. Yet we do not speak of contrition for future sins, but of caution, which is a part of prudence conjoined to penance.