A recent dispute with other commenters at Catholic and Enjoying It! has helped me to clarify my understanding of traditional moral analysis.
As has been mentioned often enough on Disputations, three factors have been identified which determine the morality of a particular human act. (A "human act" is an act freely willed by a human, and therefore either good or evil.) The factors are the object of the act, the intent of the actor, and the circumstances in which he acts.
I used to think of the object of an act as being objective -- that is, as being determined apart from any subjective input from the actor. That's true, but I was overlooking the role of the actor's will in determining the object of the act.
Of course, the actor's will must have a role, or else it wouldn't be a human act, and it would have no moral dimension at all. In fact, the will plays two formally distinct roles, one in determining the object of the act and the other in determining the intent.
How it works can be seen by considering the example of lying. "To lie," the Catechism tells us, "is to speak or act against the truth in order to lead someone into error." An argument was offered: Doesn't this mean that, by definition, the actor's intent can determine the objective moral nature of the act?
The answer is no, because the "in order to lead someone into error" is not the intent of a lie, it is the object. The intent of the lie is the end sought by the liar in telling the lie, and with the possible exception of very perverse persons, there is an end sought beyond simply leading someone into error. The actor intends this end be achieved through the means of leading someone into error.
(That, by the way, is what makes the object of an act objective. We intend the means we choose (since we, you know, choose them), and if we choose sinful means we can't deny their sinfulness simply because, subjectively, we mean to do good.)
Note what happens in the case of lying if we take out the object (which some would have be the intent): "To lie is to speak or act against the truth." But speaking against the truth is not, per se, a lie as anyone understands it; I might be mistaken, I might be talking in my sleep. More fundamentally, it is not even a meaningfully characterized human act, since it fails to characterize what the human will thinks it has chosen.
For there to be a human act, an act to which we can apply some moral value, the will has to choose to do something, and it is that choice which specifies what human act is being done.
I'd summarize the above with this table, showing different perspectives or ways of expressing the two ways the will is involved in every human act:
Object - Intent Means - End Proximate End - Remote End What human act is done? - Why is it done?