In the virtue of prudence shrewdness complements teachability and limits it. (Summa Theologica II. II. 49:4) ‘It is a disposition to acquire a right estimate by oneself’, says St. Thomas. In other words, after you have listened to the advice of those who are older and wiser, the obligation to arrive at a decision about what to do still rests on oneself. Others cannot and should not decide for us. The weight of the final decision is still a burden we must carry alone. One can be running to this or that person forever, unable to come to a decision and rest in it. Shrewdness knows when one has listened enough and found the answer; shrewdness accepts that the answer in this case might always contain some ambiguity, realises that a certain amount of risk and uncertainty must be borne, but that the issue is now as clear as it will ever be, and the time has come to act. Saint Thomas will even go so far as to say that in deliberation we may take as much time as needed, but a considered act must be performed swiftly (Summa Theologica II.II. 47:9). There comes a time, and we must simply get on with it!
It is important to remember that prudence isn’t about being indefinitely watchful and careful. Its most important act, for Saint Thomas, is the command. Prudence answers a question: “What is the best course of action in this situation?” When it discovers this answer, it commands, “Do it.” Prudence is a “directive knowledge”, for Saint Thomas.