Objection 1. It would seem that an individual action can be indifferent. For there is no species that does not, cannot, contain an individual. But an action can be indifferent in its species, as stated above (Article 8). Therefore an individual action can be indifferent.
Objection 2. Further, individual actions cause like habits, as stated in Ethic. ii, 1. But a habit can be indifferent: for the Philosopher says (Ethic. iv, 1) that those who are of an even temper and prodigal disposition are not evil; and yet it is evident that they are not good, since they depart from virtue; and thus they are indifferent in respect of a habit. Therefore some individual actions are indifferent.
Objection 3. Further, moral good belongs to virtue, while moral evil belongs to vice. But it happens sometimes that a man fails to ordain a specifically indifferent action to a vicious or virtuous end. Therefore an individual action may happen to be indifferent.
On the contrary, Gregory says in a homily (vi in Evang.): "An idle word is one that lacks either the usefulness of rectitude or the motive of just necessity or pious utility." But an idle word is an evil, because "men... shall render an account of it in the day of judgment" (Matthew 12:36): while if it does not lack the motive of just necessity or pious utility, it is good. Therefore every word is either good or bad. For the same reason every other action is either good or bad. Therefore no individual action is indifferent.
I answer that, It sometimes happens that an action is indifferent in its species, but considered in the individual it is good or evil. And the reason of this is because a moral action, as stated above (Article 3), derives its goodness not only from its object, whence it takes its species; but also from the circumstances, which are its accidents, as it were; just as something belongs to a man by reason of his individual accidents, which does not belong to him by reason of his species. And every individual action must needs have some circumstance that makes it good or bad, at least in respect of the intention of the end. For since it belongs to the reason to direct; if an action that proceeds from deliberate reason be not directed to the due end, it is, by that fact alone, repugnant to reason, and has the character of evil. But if it be directed to a due end, it is in accord with reason; wherefore it has the character of good. Now it must needs be either directed or not directed to a due end. Consequently every human action that proceeds from deliberate reason, if it be considered in the individual, must be good or bad.
If, however, it does not proceed from deliberate reason, but from some act of the imagination, as when a man strokes his beard, or moves his hand or foot; such an action, properly speaking, is not moral or human; since this depends on the reason. Hence it will be indifferent, as standing apart from the genus of moral actions.
Reply to Objection 1. For an action to be indifferent in its species can be understood in several ways. First in such a way that its species demands that it remain indifferent; and the objection proceeds along this line. But no action can be specifically indifferent thus: since no object of human action is such that it cannot be directed to good or evil, either through its end or through a circumstance. Secondly, specific indifference of an action may be due to the fact that as far as its species is concerned, it is neither good nor bad. Wherefore it can be made good or bad by something else. Thus man, as far as his species is concerned, is neither white nor black; nor is it a condition of his species that he should not be black or white; but blackness or whiteness is superadded to man by other principles than those of his species.
Reply to Objection 2. The Philosopher states that a man is evil, properly speaking, if he be hurtful to others. And accordingly, because he hurts none save himself. And the same applies to all others who are not hurtful to other men. But we say here that evil, in general, is all that is repugnant to right reason. And in this sense every individual action is either good or bad, as stated above.
Reply to Objection 3. Whenever an end is intended by deliberate reason, it belongs either to the good of some virtue, or to the evil of some vice. Thus, if a man's action is directed to the support or repose of his body, it is also directed to the good of virtue, provided he direct his body itself to the good of virtue. The same clearly applies to other actions.
Source: Summa Theologica (New Advent)