Objection 1. It would seem that a circumstance is not an accident of a human act. For Tully says (De Invent. Rhetor. i) that a circumstance is that from "which an orator adds authority and strength to his argument." But oratorical arguments are derived principally from things pertaining to the essence of a thing, such as the definition, the genus, the species, and the like, from which also Tully declares that an orator should draw his arguments. Therefore a circumstance is not an accident of a human act.
Objection 2. Further, "to be in" is proper to an accident. But that which surrounds [circumstat] is rather out than in. Therefore the circumstances are not accidents of human acts.
Objection 3. Further, an accident has no accident. But human acts themselves are accidents. Therefore the circumstances are not accidents of acts.
On the contrary, The particular conditions of any singular thing are called its individuating accidents. But the Philosopher (Ethic. iii, 1) calls the circumstances particular things [ta kath' ekasta], i.e. the particular conditions of each act. Therefore the circumstances are individual accidents of human acts.
I answer that, Since, according to the Philosopher (Peri Herm. i), "words are the signs of what we understand," it must needs be that in naming things we follow the process of intellectual knowledge. Now our intellectual knowledge proceeds from the better known to the less known. Accordingly with us, names of more obvious things are transferred so as to signify things less obvious: and hence it is that, as stated in Metaph. x, 4, "the notion of distance has been transferred from things that are apart locally, to all kinds of opposition": and in like manner words that signify local movement are employed to designate all other movements, because bodies which are circumscribed by place, are best known to us. And hence it is that the word "circumstance" has passed from located things to human acts.
Now in things located, that is said to surround something, which is outside it, but touches it, or is placed near it. Accordingly, whatever conditions are outside the substance of an act, and yet in some way touch the human act, are called circumstances. Now what is outside a thing's substance, while it belongs to that thing, is called its accident. Wherefore the circumstances of human acts should be called their accidents.
Reply to Objection 1. The orator gives strength to his argument, in the first place, from the substance of the act; and secondly, from the circumstances of the act. Thus a man becomes indictable, first, through being guilty of murder; secondly, through having done it fraudulently, or from motives of greed or at a holy time or place, and so forth. And so in the passage quoted, it is said pointedly that the orator "adds strength to his argument," as though this were something secondary.
Reply to Objection 2. A thing is said to be an accident of something in two ways. First, from being in that thing: thus, whiteness is said to be an accident of Socrates. Secondly, because it is together with that thing in the same subject: thus, whiteness is an accident of the art of music, inasmuch as they meet in the same subject, so as to touch one another, as it were. And in this sense circumstances are said to be the accidents of human acts.
Reply to Objection 3. As stated above (ad 2), an accident is said to be the accident of an accident, from the fact that they meet in the same subject. But this happens in two ways. First, in so far as two accidents are both related to the same subject, without any relation to one another; as whiteness and the art of music in Socrates. Secondly, when such accidents are related to one another; as when the subject receives one accident by means of the other; for instance, a body receives color by means of its surface. And thus also is one accident said to be in another; for we speak of color as being in the surface.
Accordingly, circumstances are related to acts in both these ways. For some circumstances that have a relation to acts, belong to the agent otherwise than through the act; as place and condition of person; whereas others belong to the agent by reason of the act, as the manner in which the act is done.
Source: Summa Theologica (New Advent)