In asking whether passions and emotions can be sinful, Aquinas raises the objection:
"No one sins in doing what he cannot avoid," as Augustine says (On the Free Choice of the Will III, 18). But man cannot escape the inordinate movement of sensuality, since "the sensuality ever remains corrupt, so long as we abide in this mortal life, and that is why it is signified by the serpent," as Augustine says (On the Trinity XII, 12,13). Therefore the inordinate movement of the sensuality is not a sin. (ST I-II, 74:3, obj. 2).
The response he makes to this objection is that though it is impossible to avoid all inordinate movements of sensitive appetite, it is possible to avoid any particular inordinate movement, and that this ability is sufficient for a voluntary sin.
[The corruption of the sensitive appetite] does not prevent man from using his rational will to suppress individual inordinate movements, if he has a presentiment of them. He can do this by, for example, turning his thoughts to other things. Yet while he is turning his thoughts to something else, an inordinate movement may arise about this also: thus when a man, in order to avoid the movements of concupiscence, turns his thoughts away from fleshly pleasures and to the consideration of science, sometimes an unforeseen (impraemeditatus) movement of vainglory will arise. And therefore a man cannot avoid all such movements, on account of the aforesaid corruption. But it is enough, for the account of a voluntary sin, that he be able to avoid each individual one. (Ibid., ad 2)
Now, if a man is in proximate danger of having an extremely disordered desire for sensitive goods, it seems clear that he ought to do what he can to avoid that, and would be guilty of neglect if he turned his attention to avoid sins into which he is in no special danger of falling. Consequently, it seems to follow from Aquinas's position that a man can in one and the same period of time have acted morally as well as he could, have made the best moral decisions that he could make, and yet be guilty of a voluntary sin. This conclusion seems, on the face of it, rather problematic.
Does Aquinas hold the same position when he considers more particular matters? It does not appear so. In a later article, he asks whether disobedience is a mortal sin, and raises the objections:
Someone is said to be disobedient when he does not fulfill his superior's command. But superiors frequently give so many commands that it is scarcely or not at all possible to keep all of them. Therefore, if disobedience were a mortal sin, it would follow that man could not avoid mortal sin, which is an untenable position. Therefore disobedience is not a mortal sin. (II-II 105:1 obj. 3)
Now, it seems equally true in this case that a person could keep any given command, and thus, by focusing on keeping the most important commands, he fails to keep some of the less important commands (whether because of time conflicts or just because there are some many commands that he can't remember all of them). It was nonetheless absolutely speaking possible for him to keep any individual one of those other commands, and thus by Aquinas's general reasoning, it would seem that the failure to keep the command remains a sin.
Aquinas does not accept the reasoning in the concrete, however, but replies:
No one is obliged to what is impossible. Therefore, if a superior gives so many commands that a subject cannot fulfill them, the subject is free of sin. And therefore superiors should refrain from giving very many commands. (Ibid, ad 3.)
I'm not sure what to think about Aquinas's position here. Is he, in an attempt to describe scientifically a real human experience, to get at the experienced psychology of such faults, making an abstract argument that is not strictly valid, and this becomes evident when one considers not abstract but concrete cases? Or is there a decisive difference between the two cases?