Objection 1. It would seem that in the angel the will does not differ from the intellect and from the nature. For an angel is more simple than a natural body. But a natural body is inclined through its form towards its end, which is its good. Therefore much more so is the angel. Now the angel's form is either the nature in which he subsists, or else it is some species within his intellect. Therefore the angel inclines towards the good through his own nature, or through an intelligible species. But such inclination towards the good belongs to the will. Therefore the will of the angel does not differ from his nature or his intellect.
Objection 2. Further, the object of the intellect is the true, while the object of the will is the good. Now the good and the true differ, not really but only logically [Cf. 16, 4]. Therefore will and intellect are not really different.
Objection 3. Further, the distinction of common and proper does not differentiate the faculties; for the same power of sight perceives color and whiteness. But the good and the true seem to be mutually related as common to particular; for the true is a particular good, to wit, of the intellect. Therefore the will, whose object is the good, does not differ from the intellect, whose object is the true.
On the contrary, The will in the angels regards good things only, while their intellect regards both good and bad things, for they know both. Therefore the will of the angels is distinct from their intellect.
I answer that, In the angels the will is a special faculty or power, which is neither their nature nor their intellect. That it is not their nature is manifest from this, that the nature or essence of a thing is completely comprised within it: whatever, then, extends to anything beyond it, is not its essence. Hence we see in natural bodies that the inclination to being does not come from anything superadded to the essence, but from the matter which desires being before possessing it, and from the form which keeps it in such being when once it exists. But the inclination towards something extrinsic comes from something superadded to the essence; as tendency to a place comes from gravity or lightness, while the inclination to make something like itself comes from the active qualities.
Now the will has a natural tendency towards good. Consequently there alone are essence and will identified where all good is contained within the essence of him who wills; that is to say, in God, Who wills nothing beyond Himself except on account of His goodness. This cannot be said of any creature, because infinite goodness is quite foreign to the nature of any created thing. Accordingly, neither the will of the angel, nor that of any creature, can be the same thing as its essence.
In like manner neither can the will be the same thing as the intellect of angel or man. Because knowledge comes about in so far as the object known is within the knower; consequently the intellect extends itself to what is outside it, according as what, in its essence, is outside it is disposed to be somehow within it. On the other hand, the will goes out to what is beyond it, according as by a kind of inclination it tends, in a manner, to what is outside it. Now it belongs to one faculty to have within itself something which is outside it, and to another faculty to tend to what is outside it. Consequently intellect and will must necessarily be different powers in every creature. It is not so with God, for He has within Himself universal being, and the universal good. Therefore both intellect and will are His nature.
Reply to Objection 1. A natural body is moved to its own being by its substantial form: while it is inclined to something outside by something additional, as has been said.
Reply to Objection 2. Faculties are not differentiated by any material difference of their objects, but according to their formal distinction, which is taken from the nature of the object as such. Consequently the diversity derived from the notion of good and true suffices for the difference of intellect from will.
Reply to Objection 3. Because the good and the true are really convertible, it follows that the good is apprehended by the intellect as something true; while the true is desired by the will as something good. Nevertheless, the diversity of their aspects is sufficient for diversifying the faculties, as was said above (ad 2).
Source: Summa Theologica (New Advent)