Objection 1. It would seem that the circumstances are not properly set forth in Ethic. iii, 1. For a circumstance of an act is described as something outside the act. Now time and place answer to this description. Therefore there are only two circumstances, to wit, "when" and "where."
Objection 2. Further, we judge from the circumstances whether a thing is well or ill done. But this belongs to the mode of an act. Therefore all the circumstances are included under one, which is the "mode of acting."
Objection 3. Further, circumstances are not part of the substance of an act. But the causes of an act seem to belong to its substance. Therefore no circumstance should be taken from the cause of the act itself. Accordingly, neither "who," nor "why," nor "about what," are circumstances: since "who" refers to the efficient cause, "why" to the final cause, and "about what" to the material cause.
On the contrary is the authority of the Philosopher in Ethic. iii, 1.
I answer that, Tully, in his Rhetoric (De Invent. Rhetor. i), gives seven circumstances, which are contained in this verse:
Who, what, where, by what aids, why, how, and when."
Aristotle
The reason of this enumeration may be set down as follows. For a circumstance is described as something outside the substance of the act, and yet in a way touching it. Now this happens in three ways: first, inasmuch as it touches the act itself; secondly, inasmuch as it touches the cause of the act; thirdly, inasmuch as it touches the effect. It touches the act itself, either by way of measure, as "time" and "place"; or by qualifying the act as the "mode of acting." It touches the effect when we consider "what" is done. It touches the cause of the act, as to the final cause, by the circumstance "why"; as to the material cause, or object, in the circumstance "about what"; as to the principal efficient cause, in the circumstance "who"; and as to the instrumental efficient cause, in the circumstance "by what aids."
Reply to Objection 1. Time and place surround [circumstant] the act by way of measure; but the others surround the act by touching it in any other way, while they are extrinsic to the substance of the act.
Reply to Objection 2. This mode "well" or "ill" is not a circumstance, but results from all the circumstances. But the mode which refers to a quality of the act is a special circumstance; for instance, that a man walk fast or slowly; that he strike hard or gently, and so forth.
Reply to Objection 3. A condition of the cause, on which the substance of the act depends, is not a circumstance; it must be an additional condition. Thus, in regard to the object, it is not a circumstance of theft that the object is another's property, for this belongs to the substance of the act; but that it be great or small. And the same applies to the other circumstances which are considered in reference to the other causes. For the end that specifies the act is not a circumstance, but some additional end. Thus, that a valiant man act "valiantly for the sake of" the good of the virtue or fortitude, is not a circumstance; but if he act valiantly for the sake of the delivery of the state, or of Christendom, or some such purpose. The same is to be said with regard to the circumstance "what"; for that a man by pouring water on someone should happen to wash him, is not a circumstance of the washing; but that in doing so he give him a chill, or scald him; heal him or harm him, these are circumstances.
Source: Summa Theologica (New Advent)