I should have done this a while ago, but... well, you get what you pay for.
Anyway, what I'm trying to do is work my way through the section of St. Thomas's Summa Theologiae that deals with acts that are proper to man. (If I finish that, we'll see whether I go on to the second part of the full Treatise on Human Acts, which deals with the passions that are proper to both man and other animals.)
This two-part discussion of human acts comes immediately after the discussion of human happiness (ST I-II, 1-5). St. Thomas explains why:
Since therefore Happiness must be gained by means of certain acts, we are obliged consequently to consider human acts, in order to know by what acts we may obtain Happiness, and by what acts we are prevented from obtaining it.
There, in forty words, is the explanation for studying morality. Not to pass an examination, not to beguile the hours with a pleasant pastime, but to obtain Happiness.
He goes on:
And since Happiness is man's proper good, those acts which are proper to man have a closer connection with Happiness than have those which are common to man and the other animals.
So he starts with Acts Proper to Man.
His discussion is structured thusly (click to enlarge):
So far, I've blogged on qq 6-7. As you recall, in q 6, a 4, St. Thomas teaches that
The act of the will is twofold: one is its immediate act, as it were, elicited by it, namely, "to wish"; the other is an act of the will commanded by it, and put into execution by means of some other power, such as "to walk" and "to speak," which are commanded by the will to be executed by means of the motive power.
The "immediate act" elicited by the will is discussed in qq 8-16:
The act of the will that commands other powers -- that is, that causes you to actually do something beyond merely wishing to do it -- is covered in q 17.
As you can see from the second diagram, St. Thomas teaches that we can wish an end three different ways: acts of volition; acts of enjoyment; and acts of intention. Right now, I'm one article into the three questions that treat the will's immediate act of volition.
(And, at the risk of spoiling things, I can say that the answer to the question in the first diagram, "What distinguishes human acts?" is whether an act is good or evil.)