Objection 1. It would seem that fear causes involuntariness simply. For just as violence regards that which is contrary to the will at the time, so fear regards a future evil which is repugnant to the will. But violence causes involuntariness simply. Therefore fear too causes involuntariness simply.
Objection 2. Further, that which is such of itself, remains such, whatever be added to it: thus what is hot of itself, as long as it remains, is still hot, whatever be added to it. But that which is done through fear, is involuntary in itself. Therefore, even with the addition of fear, it is involuntary.
Objection 3. Further, that which is such, subject to a condition, is such in a certain respect; whereas what is such, without any condition, is such simply: thus what is necessary, subject to a condition, is necessary in some respect: but what is necessary absolutely, is necessary simply. But that which is done through fear, is absolutely involuntary; and is not voluntary, save under a condition, namely, in order that the evil feared may be avoided. Therefore that which is done through fear, is involuntary simply.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xxx.] and the Philosopher (Ethic. iii, 1) say that such things as are done through fear are "voluntary rather than involuntary."
I answer that, As the Philosopher says (Ethic. iii) and likewise Gregory of Nyssa in his book on Man (Nemesius, De Nat. Hom. xxx), such things are done through fear "are of a mixed character," being partly voluntary and partly involuntary. For that which is done through fear, considered in itself, is not voluntary; but it becomes voluntary in this particular case, in order, namely, to avoid the evil feared.
But if the matter be considered aright, such things are voluntary rather than involuntary; for they are voluntary simply, but involuntary in a certain respect. For a thing is said to be simply, according as it is in act; but according as it is only in apprehension, it is not simply, but in a certain respect. Now that which is done through fear, is in act in so far as it is done. For, since acts are concerned with singulars; and the singular, as such, is here and now; that which is done is in act, in so far as it is here and now and under other individuating circumstances. And that which is done through fear is voluntary, inasmuch as it is here and now, that is to say, in so far as, under the circumstances, it hinders a greater evil which was feared; thus the throwing of the cargo into the sea becomes voluntary during the storm, through fear of the danger: wherefore it is clear that it is voluntary simply. And hence it is that what is done out of fear is essentially voluntary, because its principle is within. But if we consider what is done through fear, as outside this particular case, and inasmuch as it is repugnant to the will, this is merely a consideration of the mind. And consequently what is done through fear is involuntary, considered in that respect, that is to say, outside the actual circumstances of the case.
Reply to Objection 1. Things done through fear and compulsion differ not only according to present and future time, but also in this, that the will does not consent, but is moved entirely counter to that which is done through compulsion: whereas what is done through fear, becomes voluntary, because the will is moved towards it, albeit not for its own sake, but on account of something else, that is, in order to avoid an evil which is feared. For the conditions of a voluntary act are satisfied, if it be done on account of something else voluntary: since the voluntary is not only what we wish, for its own sake, as an end, but also what we wish for the sake of something else, as an end. It is clear therefore that in what is done from compulsion, the will does nothing inwardly; whereas in what is done through fear, the will does something. Accordingly, as Gregory of Nyssa [Nemesius, De Nat. Hom. xxx.] says, in order to exclude things done through fear, a violent action is defined as not only one, "the principal whereof is from without," but with the addition, "in which he that suffers violence concurs not at all"; because the will of him that is in fear, does concur somewhat in that which he does through fear.
Reply to Objection 2. Things that are such absolutely, remain such, whatever be added to them; for instance, a cold thing, or a white thing: but things that are such relatively, vary according as they are compared with different things. For what is big in comparison with one thing, is small in comparison with another. Now a thing is said to be voluntary, not only for its own sake, as it were absolutely; but also for the sake of something else, as it were relatively. Accordingly, nothing prevents a thing which was not voluntary in comparison with one thing, from becoming voluntary when compared with another.
Reply to Objection 3. That which is done through fear, is voluntary without any condition, that is to say, according as it is actually done: but it is involuntary, under a certain condition, that is to say, if such a fear were not threatening. Consequently, this argument proves rather the opposite.
Source: Summa Theologica (New Advent)