I’m being told that pushing someone off a lifeboat far from land is morally neutral. Of course it isn’t.
Make the distinctions, pick up on the subtleties, and be sure you respond to what is actually said.
I suggest your re-read your Summa citations. Aquinas does not teach that circumstances change the object of an act. Nor does he teach that circumstances “speciate” the object. Your confusion, I believe, is in equivocating Aquinas’ phrases “moral species” and “moral action.” The former references the object and the latter references the action.
The object is essential; circumstances are accidental. The accidents cannot change the essential genus of the object but they do change the the morality of the action.
They change the morality of the act by changing the object. He says that actions take their species from their object. He says that circumstances can change the object if they enter into the essence. Let’s go through what you quoted. My emphases and comments… follow the *'s too.
Article 3. Whether man’s action is good or evil from a circumstance?
Reply to Objection 1. Circumstances are outside an action,
inasmuch as they are not part of its essence [which means sometimes they can be]; but they are in an action as accidents thereof. Thus, too, accidents in natural substances are outside the essence.
Article 8. Whether any action is indifferent in its species?
I answer that, As stated above (2,5), every action takes its species from its object; while human action, which is called moral,
takes its species from the object, in relation to the principle of human actions, which is the reason. [And sometimes reason* is bound to consider the circumstances in the action.]
Article 10. Whether a circumstance places a moral action in the species of good or evil?
On the contrary, Place is a circumstance. But place makes a moral action to be in a certain species of evil; for theft of a thing from a holy place is a sacrilege. Therefore a circumstance makes a moral action to be specifically good or bad. And in this way,
whenever a circumstance has a special relation to reason, either for or against, it must needs specify the moral action whether good or bad. [When your reason* ought to know that what you are doing is bad precisely because of the circumstance.]
Hence it is that in natural things, that which is accidental to a thing, cannot be taken as a difference constituting the species.
But the process of reason is not fixed to one particular term, for at any point it can still proceed further. [Like we can think about our arms moving, or that our arms are connected to a tennis racket, and that we are in a church, etc.] And consequently that which, in one action,
is taken as a circumstance added to the object that specifies the action, can again be taken by the
directing reason [huzzah!*],
as the principal condition of the object that determines the action’s species." [The circumstance can change the object, the object can change the goodness of the act.]
I have said circumstances “speciate” the object (or specify if you prefer), because the object is specifying the whole act… since it really is the whole act. The distinction between them in Thomas is this: objects are specific kinds of acts (murder, almsgiving, etc.) and actions are good, bad, or neutral. The intention is only the motivation for the act and makes the action bad by association. The circumstances are what surrounds the act and don’t matter unless they enter into the object.
You want “circumstance” to sit alone and remain its own little aspect of a moral evaluation, but that’s just not what happens. It is useful to distinguish it at the start of an evaluation only insofar as it clarifies what is being brought to bear on one’s intuitive conception of the more basic action being evaluated.