Human acts are actions willed by human persons, so it makes sense (and in practice has proved useful) to categorize human acts according to the things that are desired by the will. The things directly desired, to be more precise, so that you can distinguish between two human acts that follow different means to the same (remote) end.
The thing directly desired, the proximate end of a human act, is the object of the act. We can say we specify human acts by their objects, or if we're feeling fancy we can say the object determines an act's species.
The morality of a particular human act is fully determined by its object, the circumstances in which it is done, and the intention of the actor (the remote end or the reason why he wants the object of his act).
Nothing new here, and I've been quoting the Catechism on this for years.
But there's a wrinkle in this that I've also been trying to iron out for years. A lot of things that we think of as kinds of human acts seem to be defined in terms of circumstances and intention as well as object. They are, in effect, subspecies of acts that are specified by their objects alone.
Gluttony, for example, seems like a perfect example. St. Thomas defines it as simply an inordinate desire of eating and drinking. But surely whether a particular meal or drink is "inordinate" depends on circumstances and intentions -- how much food and drink you've had recently, for example, or whether you're an athlete in training.
St. Thomas addresses precisely this issue. He follows St. Gregory on the five species of gluttony, which you can remember with this little verse:
praepropere, laute, nimis, ardenter, studiose
In plain English, gluttony can consist in eating
Hastily, sumptuously, too much, greedily, [or] daintily
The first objection to this list of the specific kinds of gluttony is that
the above are distinguished according to diversity of circumstance. Now circumstances, being the accidents of an act, do not differentiate its species.
Say, that's just what I said!
St. Thomas responds:
The corruption of various circumstances causes the various species of gluttony, on account of the various motives, by reason of which the species of moral things are differentiated. For in him that seeks sumptuous food, concupiscence is aroused by the very species of the food; in him that forestalls the time concupiscence is disordered through impatience of delay, and so forth.
"Motives" here does not mean intentions or remote ends, since intentions don't differentiate species of acts any more than circumstances do.
Rather, it refers to what it is about eating and drinking that the glutton directly desires. The object of the sumptuous glutton's act is eating foods insofar as they are costly; if the food he eats isn't expensive, he is not doing what he wills to do.
the process of reason is not fixed to one particular term, for at any point it can still proceed further. And consequently that which, in one action, is taken as a circumstance added to the object that specifies the action, can again be taken by the directing reason, as the principal condition of the object that determines the action's species.
It turns out, then, that the object of a human act can be an accident of a more general human act.
And this isn't a sleight of hand so that moral theologians can say that circumstances don't differentiate species of acts, except when they do. It's a recognition that humans actually can and do turn corrupt circumstances into objects of desire, that we can and do sometimes choose things like this-toy-airplane-that-belongs-to-my-older-brother and not merely like this-toy-airplane, which belongs to my older brother.
(We also turn good or indifferent circumstances into objects of desire, but since circumstances can't make evil acts good -- whether you take a circumstance as the principal condition of the object or not -- St. Thomas isn't concerned with that in these articles.)