Objection 1. It would seem that the will is not of good only. For the same power regards opposites; for instance, sight regards white and black. But good and evil are opposites. Therefore the will is not only of good, but also of evil.
Objection 2. Further, rational powers can be directed to opposite purposes, according to the Philosopher (Metaph. ix, 2). But the will is a rational power, since it is "in the reason," as is stated in De Anima iii, 9. Therefore the will can be directed to opposites; and consequently its volition is not confined to good, but extends to evil.
Objection 3. Further, good and being are convertible. But volition is directed not only to beings, but also to non-beings. For sometimes we wish "not to walk," or "not to speak"; and again at times we wish for future things, which are not actual beings. Therefore the will is not of good only.
On the contrary, Dionysius says (Div. Nom. iv) that "evil is outside the scope of the will," and that "all things desire good."
I answer that, The will is a rational appetite. Now every appetite is only of something good. The reason of this is that the appetite is nothing else than an inclination of a person desirous of a thing towards that thing. Now every inclination is to something like and suitable to the thing inclined. Since, therefore, everything, inasmuch as it is being and substance, is a good, it must needs be that every inclination is to something good. And hence it is that the Philosopher says (Ethic. i, 1) that "the good is that which all desire."
But it must be noted that, since every inclination results from a form, the natural appetite results from a form existing in the nature of things: while the sensitive appetite, as also the intellective or rational appetite, which we call the will, follows from an apprehended form. Therefore, just as the natural appetite tends to good existing in a thing; so the animal or voluntary appetite tends to a good which is apprehended. Consequently, in order that the will tend to anything, it is requisite, not that this be good in very truth, but that it be apprehended as good. Wherefore the Philosopher says (Phys. ii, 3) that "the end is a good, or an apparent good."
Reply to Objection 1. The same power regards opposites, but it is not referred to them in the same way. Accordingly, the will is referred both to good and evil: but to good by desiring it: to evil, by shunning it. Wherefore the actual desire of good is called "volition" [In Latin, 'voluntas'. To avoid confusion with "voluntas" (the will) St. Thomas adds a word of explanation, which in the translation may appear superfluous], meaning thereby the act of the will; for it is in this sense that we are now speaking of the will. On the other hand, the shunning of evil is better described as "nolition": wherefore, just as volition is of good, so nolition is of evil.
Reply to Objection 2. A rational power is not to be directed to all opposite purposes, but to those which are contained under its proper object; for no power seeks other than its proper object. Now, the object of the will is good. Wherefore the will can be directed to such opposite purposes as are contained under good, such as to be moved or to be at rest, to speak or to be silent, and such like: for the will can be directed to either under the aspect of good.
Reply to Objection 3. That which is not a being in nature, is considered as a being in the reason, wherefore negations and privations are said to be "beings of reason." In this way, too, future things, in so far as they are apprehended, are beings. Accordingly, in so far as such like are beings, they are apprehended under the aspect of good; and it is thus that the will is directed to them. Wherefore the Philosopher says (Ethic. v, 1) that "to lack evil is considered as a good."
Source: Summa Theologica (New Advent)