I wasn’t terribly pleased with some of my examples of the parts of prudence from my blizzard story. So let me take another crack at it by setting down how St. Thomas thinks the mother of the virtues works (and then maybe try to work back to better examples).
His paradigm for means-end reasoning is the practical syllogism. The end serves as one premise; other premises are either universal principles, conclusions of other sciences, or what lies close at hand–things we can actually do now. The conclusion of the practical syllogism is the choice we are to make, which we then must execute to bring the end about.
[As an aside, this is how Aquinas thinks our will is free: the conclusion of the practical syllogism, unlike in a deduction, does not follow of necessity. Don’t worry, I’m not going to tango with free will in this blog post.]
Prudence is the virtue that perfects this process of counsel-consent-choice. The eight parts of prudence all perfect some aspect of syllogizing:
Memoria is knowledge of what worked and didn’t work as a premise “last time.”
Intellectum is knowledge of those universal principles we need as premises.
Docilitas is a willingness to receive some of those principles from other people since we know we don’t know everything.
Solertia is the ability to come up with a middle term to complete the syllogism quickly.
Ratio is the ability to follow the premises “to their logical conclusion”–in this case, choosing and acting.
Providentia is the ability to see in advance which close-at-hand premises you will need to complete the syllogism.
Circumspection is the ability to see which premises, though good for me reaching my end, will mix badly with other circumstances and produce evils in someone else’s syllogism.
Cautio is the ability to see which premises can or will bring evils with them that hinder me from completing the syllogism.
The ones that are really syllogistic–or rather, broadly syllogistic–are intellectum, solertia, and ratio: understanding premises, coming up with solutions quickly, and following premises to their logical conclusion. The others are more specific to practical syllogisms and don’t cross over to Barbara deductions (or whatever) quite so neatly.
Another thing I forgot to mention in the earlier post is how interlocked the parts are. One of my students put it as “bleeding together.” Our experience of foresight is clearly grounded in memory of what has worked or not worked in the past, both of those give rise to quick thinking in the face of surprising situations, etc. Memory is, in a way, the foundation of all of prudence, and foresight is its pinnacle. The word prudence is, after all, just a contraction of the word providentia.
Finally, the parts of prudence get carved up differently in many authors. It’s easy to see how circumspection and caution are very similar, and further can roll into foresight, for example. Likewise solertia and ratio are very close. Aquinas devotes an article to looking at several different authors and explaining why they carve up the subject the way they do.
Amusingly, Pieper–a German academic–thinks Aquinas has played a few too many distinctions and collapses the parts back down to four (or five, depending on how you want to think about synderesis). When a German thinks you have gone to far with the system-building, look out!