Objection 1. It would seem that the will is not moved to anything naturally. For the natural agent is condivided with the voluntary agent, as stated at the beginning of Phys. ii, 1. Therefore the will is not moved to anything naturally.
Objection 2. Further, that which is natural is in a thing always: as "being hot" is in fire. But no movement is always in the will. Therefore no movement is natural to the will.
Objection 3. Further, nature is determinate to one thing: whereas the will is referred to opposites. Therefore the will wills nothing naturally.
On the contrary, The movement of the will follows the movement of the intellect. But the intellect understands some things naturally. Therefore the will, too, wills some things naturally.
I answer that, As Boethius says (De Duabus Nat.) and the Philosopher also (Metaph. v, 4) the word "nature" is used in a manifold sense. For sometimes it stands for the intrinsic principle in movable things. In this sense nature is either matter or the material form, as stated in Phys. ii, 1. In another sense nature stands for any substance, or even for any being. And in this sense, that is said to be natural to a thing which befits it in respect of its substance. And this is that which of itself is in a thing. Now all things that do not of themselves belong to the thing in which they are, are reduced to something which belongs of itself to that thing, as to their principle. Wherefore, taking nature in this sense, it is necessary that the principle of whatever belongs to a thing, be a natural principle. This is evident in regard to the intellect: for the principles of intellectual knowledge are naturally known. In like manner the principle of voluntary movements must be something naturally willed.
Now this is good in general, to which the will tends naturally, as does each power to its object; and again it is the last end, which stands in the same relation to things appetible, as the first principles of demonstrations to things intelligible: and, speaking generally, it is all those things which belong to the willer according to his nature. For it is not only things pertaining to the will that the will desires, but also that which pertains to each power, and to the entire man. Wherefore man wills naturally not only the object of the will, but also other things that are appropriate to the other powers; such as the knowledge of truth, which befits the intellect; and to be and to live and other like things which regard the natural well-being; all of which are included in the object of the will, as so many particular goods.
Reply to Objection 1. The will is distinguished from nature as one kind of cause from another; for some things happen naturally and some are done voluntarily. There is, however, another manner of causing that is proper to the will, which is mistress of its act, besides the manner proper to nature, which is determinate to one thing. But since the will is founded on some nature, it is necessary that the movement proper to nature be shared by the will, to some extent: just as what belongs to a previous cause is shared by a subsequent cause. Because in every thing, being itself, which is from nature, precedes volition, which is from the will. And hence it is that the will wills something naturally.
Reply to Objection 2. In the case of natural things, that which is natural, as a result of the form only, is always in them actually, as heat is in fire. But that which is natural as a result of matter, is not always in them actually, but sometimes only in potentiality: because form is act, whereas matter is potentiality. Now movement is "the act of that which is in potentiality" (Aristotle, Phys. iii, 1). Wherefore that which belongs to, or results from, movement, in regard to natural things, is not always in them. Thus fire does not always move upwards, but only when it is outside its own place. [The Aristotelian theory was that fire's proper place is the fiery heaven, i.e. the Empyrean.] And in like manner it is not necessary that the will (which is reduced from potentiality to act, when it wills something), should always be in the act of volition; but only when it is in a certain determinate disposition. But God's will, which is pure act, is always in the act of volition.
Reply to Objection 3. To every nature there is one thing corresponding, proportionate, however, to that nature. For to nature considered as a genus, there corresponds something one generically; and to nature as species there corresponds something one specifically; and to the individualized nature there corresponds some one individual. Since, therefore, the will is an immaterial power like the intellect, some one general thing corresponds to it, naturally which is the good; just as to the intellect there corresponds some one general thing, which is the true, or being, or "what a thing is." And under good in general are included many particular goods, to none of which is the will determined.
Source: Summa Theologica (New Advent)