Objection 1. It would seem that voluntariness cannot be without any act. For that is voluntary which proceeds from the will. But nothing can proceed from the will, except through some act, at least an act of the will. Therefore there cannot be voluntariness without act.
Objection 2. Further, just as one is said to wish by an act of the will, so when the act of the will ceases, one is said not to wish. But not to wish implies involuntariness, which is contrary to voluntariness. Therefore there can be nothing voluntary when the act of the will ceases.
Objection 3. Further, knowledge is essential to the voluntary, as stated above (1,2). But knowledge involves an act. Therefore voluntariness cannot be without some act.
On the contrary, The word "voluntary" is applied to that of which we are masters. Now we are masters in respect of to act and not to act, to will and not to will. Therefore just as to act and to will are voluntary, so also are not to act and not to will.
I answer that, Voluntary is what proceeds from the will. Now one thing proceeds from another in two ways. First, directly; in which sense something proceeds from another inasmuch as this other acts; for instance, heating from heat. Secondly, indirectly; in which sense something proceeds from another through this other not acting; thus the sinking of a ship is set down to the helmsman, from his having ceased to steer. But we must take note that the cause of what follows from want of action is not always the agent as not acting; but only then when the agent can and ought to act. For if the helmsman were unable to steer the ship or if the ship's helm be not entrusted to him, the sinking of the ship would not be set down to him, although it might be due to his absence from the helm.
Since, then, the will by willing and acting, is able, and sometimes ought, to hinder not-willing and not-acting; this not-willing and not-acting is imputed to, as though proceeding from, the will. And thus it is that we can have the voluntary without an act; sometimes without outward act, but with an interior act; for instance, when one wills not to act; and sometimes without even an interior act, as when one does not will to act.
Reply to Objection 1. We apply the word "voluntary" not only to that which proceeds from the will directly, as from its action; but also to that which proceeds from it indirectly as from its inaction.
Reply to Objection 2. "Not to wish" is said in two senses. First, as though it were one word, and the infinitive of "I-do-not-wish." Consequently just as when I say "I do not wish to read," the sense is, "I wish not to read"; so "not to wish to read" is the same as "to wish not to read," and in this sense "not to wish" implies involuntariness. Secondly it is taken as a sentence: and then no act of the will is affirmed. And in this sense "not to wish" does not imply involuntariness.
Reply to Objection 3. Voluntariness requires an act of knowledge in the same way as it requires an act of will; namely, in order that it be in one's power to consider, to wish and to act. And then, just as not to wish, and not to act, when it is time to wish and to act, is voluntary, so is it voluntary not to consider.
Source: Summa Theologica (New Advent)