So then, where does St. Thomas begin when he looks at the virtue of prudence? For him, the first aspect of prudence is memory (see Summa Theologica, II.II: 49:1). Why? Because
…it is typical of prudence to be aware of what is true in the majority of cases. This kind of awareness is fostered and engendered by experience and time, therefore, prudence requires the memory of many things.
Perhaps it is easier to understand this by looking at the opposite quality. I suspect we all know someone about whom others will roll their eyes and sigh, saying, “Oh dear. Jack never learns.” Here, Jack is someone who makes the same big mistakes over and over: the small business person, say, who hires incompetent and dishonest employees out of a desire to help the under-dog. These employees subsequently harm the business through irresponsibility or theft. This becomes a pattern, though, in Jack’s business career. He lets his need to “save” people who have a sob story get in the way of his judgement. Repeatedly.
It is the repetition of the error that is at issue here. Memory, says Thomas, is aided by diligence. With diligence, we make a mental note of what happens, we put conscious effort into noticing how events unfold in matters that are important to us. We don’t just let life go by, and let the same mistakes happen again and again. We ask why something keeps happening. From this, we gain some capacity to predict what is likely to happen if we do the same thing again. ‘It behoves us to argue about the future from the past; therefore memory of the past is necessary in order to take good counsel for the future,’ says Saint Thomas.