Objection 1. It would seem that memory is not a part of prudence. For memory, as the Philosopher proves (De Memor. et Remin. i), is in the sensitive part of the soul: whereas prudence is in the rational part (Ethic. vi, 5). Therefore memory is not a part of prudence.
Objection 2. Further, prudence is acquired and perfected by experience, whereas memory is in us from nature. Therefore memory is not a part of prudence.
Objection 3. Further, memory regards the past, whereas prudence regards future matters of action, about which counsel is concerned, as stated in Ethic. vi, 2,7. Therefore memory is not a part of prudence.
On the contrary, Tully (De Invent. Rhet. ii, 53) places memory among the parts of prudence.
I answer that, Prudence regards contingent matters of action, as stated above (Question 47, Article 5). Now in such like matters a man can be directed, not by those things that are simply and necessarily true, but by those which occur in the majority of cases: because principles must be proportionate to their conclusions, and "like must be concluded from like" (Ethic. vi [Anal. Post. i. 32). But we need experience to discover what is true in the majority of cases: wherefore the Philosopher says (Ethic. ii, 1) that "intellectual virtue is engendered and fostered by experience and time." Now experience is the result of many memories as stated in Metaph. i, 1, and therefore prudence requires the memory of many things. Hence memory is fittingly accounted a part of prudence.
Reply to Objection 1. As stated above (47, 3,6), prudence applies universal knowledge to particulars which are objects of sense: hence many things belonging to the sensitive faculties are requisite for prudence, and memory is one of them.
Reply to Objection 2. Just as aptitude for prudence is in our nature, while its perfection comes through practice or grace, so too, as Tully says in his Rhetoric [Ad Herenn. de Arte Rhet. iii, 16,24, memory not only arises from nature, but is also aided by art and diligence.
There are four things whereby a man perfects his memory. First, when a man wishes to remember a thing, he should take some suitable yet somewhat unwonted illustration of it, since the unwonted strikes us more, and so makes a greater and stronger impression on the mind; the mind; and this explains why we remember better what we saw when we were children. Now the reason for the necessity of finding these illustrations or images, is that simple and spiritual impressions easily slip from the mind, unless they be tied as it were to some corporeal image, because human knowledge has a greater hold on sensible objects. For this reason memory is assigned to the sensitive part of the soul. Secondly, whatever a man wishes to retain in his memory he must carefully consider and set in order, so that he may pass easily from one memory to another. Hence the Philosopher says (De Memor. et Remin. ii): "Sometimes a place brings memories back to us: the reason being that we pass quickly from the one to the other." Thirdly, we must be anxious and earnest about the things we wish to remember, because the more a thing is impressed on the mind, the less it is liable to slip out of it. Wherefore Tully says in his Rhetoric [Ad Herenn. de Arte Rhet. iii.] that "anxiety preserves the figures of images entire." Fourthly, we should often reflect on the things we wish to remember. Hence the Philosopher says (De Memoria i) that "reflection preserves memories," because as he remarks (De Memoria ii) "custom is a second nature": wherefore when we reflect on a thing frequently, we quickly call it to mind, through passing from one thing to another by a kind of natural order.
Reply to Objection 3. It behooves us to argue, as it were, about the future from the past; wherefore memory of the past is necessary in order to take good counsel for the future.
Source: Summa Theologica (New Advent)