Objection 1. It would seem that circumspection cannot be a part of prudence. For circumspection seems to signify looking at one's surroundings. But these are of infinite number, and cannot be considered by the reason wherein is prudence. Therefore circumspection should not be reckoned a part of prudence.
Objection 2. Further, circumstances seem to be the concern of moral virtues rather than of prudence. But circumspection seems to denote nothing but attention to circumstances. Therefore circumspection apparently belongs to the moral virtues rather than to prudence.
Objection 3. Further, whoever can see things afar off can much more see things that are near. Now foresight enables a man to look on distant things. Therefore there is no need to account circumspection a part of prudence in addition to foresight.
On the contrary stands the authority of Macrobius, quoted above (Article 48).
I answer that, As stated above (Article 6), it belongs to prudence chiefly to direct something aright to an end; and this is not done aright unless both the end be good, and the means good and suitable.
Since, however, prudence, as stated above (Question 47, Article 3) is about singular matters of action, which contain many combinations of circumstances, it happens that a thing is good in itself and suitable to the end, and nevertheless becomes evil or unsuitable to the end, by reason of some combination of circumstances. Thus to show signs of love to someone seems, considered in itself, to be a fitting way to arouse love in his heart, yet if pride or suspicion of flattery arise in his heart, it will no longer be a means suitable to the end. Hence the need of circumspection in prudence, viz. of comparing the means with the circumstances.
Reply to Objection 1. Though the number of possible circumstances be infinite, the number of actual circumstances is not; and the judgment of reason in matters of action is influenced by things which are few in number
Reply to Objection 2. Circumstances are the concern of prudence, because prudence has to fix them; on the other hand they are the concern of moral virtues, in so far as moral virtues are perfected by the fixing of circumstances.
Reply to Objection 3. Just as it belongs to foresight to look on that which is by its nature suitable to an end, so it belongs to circumspection to consider whether it be suitable to the end in view of the circumstances. Now each of these presents a difficulty of its own, and therefore each is reckoned a distinct part of prudence.
Source: Summa Theologica (New Advent)