Objection 1. It would seem that these are not the most important circumstances, namely, "why" and those "in which the act is, [hen ois e praxis]" as stated in Ethic. iii, 1. For those in which the act is seem to be place and time: and these do not seem to be the most important of the circumstances, since, of them all, they are the most extrinsic to the act. Therefore those things in which the act is are not the most important circumstances.
Objection 2. Further, the end of a thing is extrinsic to it. Therefore it is not the most important circumstance.
Objection 3. Further, that which holds the foremost place in regard to each thing, is its cause and its form. But the cause of an act is the person that does it; while the form of an act is the manner in which it is done. Therefore these two circumstances seem to be of the greatest importance.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xxxi.] says that "the most important circumstances" are "why it is done" and "what is done."
I answer that, As stated above (Question 1, Article 1), acts are properly called human, inasmuch as they are voluntary. Now, the motive and object of the will is the end. Therefore that circumstance is the most important of all which touches the act on the part of the end, viz. the circumstance "why": and the second in importance, is that which touches the very substance of the act, viz. the circumstance "what he did." As to the other circumstances, they are more or less important, according as they more or less approach to these.
Reply to Objection 1. By those things "in which the act is" the Philosopher does not mean time and place, but those circumstances that are affixed to the act itself. Wherefore Gregory of Nyssa [Nemesius, De Nat. Hom. xxx.], as though he were explaining the dictum of the Philosopher, instead of the latter's term--"in which the act is"--said, "what is done."
Reply to Objection 2. Although the end is not part of the substance of the act, yet it is the most important cause of the act, inasmuch as it moves the agent to act. Wherefore the moral act is specified chiefly by the end.
Reply to Objection 3. The person that does the act is the cause of that act, inasmuch as he is moved thereto by the end; and it is chiefly in this respect that he is directed to the act; while other conditions of the person have not such an important relation to the act. As to the mode, it is not the substantial form of the act, for in an act the substantial form depends on the object and term or end; but it is, as it were, a certain accidental quality of the act.
Source: Summa Theologica (New Advent)