Objection 1. It would seem that caution should not be reckoned a part of prudence. For when no evil is possible, no caution is required. Now no man makes evil use of virtue, as Augustine declares (De Lib. Arb. ii, 19). Therefore caution does not belong to prudence which directs the virtues.
Objection 2. Further, to foresee good and to avoid evil belong to the same faculty, just as the same art gives health and cures ill-health. Now it belongs to foresight to foresee good, and consequently, also to avoid evil. Therefore caution should not be accounted a part of prudence, distinct from foresight.
Objection 3. Further, no prudent man strives for the impossible. But no man can take precautions against all possible evils. Therefore caution does not belong to prudence.
On the contrary, The Apostle says (Ephesians 5:15): "See how you walk cautiously [Douay: 'circumspectly']."
I answer that, The things with which prudence is concerned, are contingent matters of action, wherein, even as false is found with true, so is evil mingled with good, on account of the great variety of these matters of action, wherein good is often hindered by evil, and evil has the appearance of good. Wherefore prudence needs caution, so that we may have such a grasp of good as to avoid evil.
Reply to Objection 1. Caution is required in moral acts, that we may be on our guard, not against acts of virtue, but against the hindrance of acts of virtue.
Reply to Objection 2. It is the same in idea, to ensue good and to avoid the opposite evil, but the avoidance of outward hindrances is different in idea. Hence caution differs from foresight, although they both belong to the one virtue of prudence.
Reply to Objection 3. Of the evils which man has to avoid, some are of frequent occurrence; the like can be grasped by reason, and against them caution is directed, either that they may be avoided altogether, or that they may do less harm. Others there are that occur rarely and by chance, and these, since they are infinite in number, cannot be grasped by reason, nor is man able to take precautions against them, although by exercising prudence he is able to prepare against all the surprises of chance, so as to suffer less harm thereby.
Source: Summa Theologica (New Advent)