Objection 1. It would seem that the will is a higher power than the intellect. For the object of the will is good and the end. But the end is the first and highest cause. Therefore the will is the first and highest power.
Objection 2. Further, in the order of natural things we observe a progress from imperfect things to perfect. And this also appears in the powers of the soul: for sense precedes the intellect, which is more noble. Now the act of the will, in the natural order, follows the act of the intellect. Therefore the will is a more noble and perfect power than the intellect.
Objection 3. Further, habits are proportioned to their powers, as perfections to what they make perfect. But the habit which perfects the will--namely, charity--is more noble than the habits which perfect the intellect: for it is written (1 Corinthians 13:2): "If I should know all mysteries, and if I should have all faith, and have not charity, I am nothing." Therefore the will is a higher power than the intellect.
On the contrary, The Philosopher holds the intellect to be the higher power than the will.
I answer that, The superiority of one thing over another can be considered in two ways: "absolutely" and "relatively." Now a thing is considered to be such absolutely which is considered such in itself: but relatively as it is such with regard to something else. If therefore the intellect and will be considered with regard to themselves, then the intellect is the higher power. And this is clear if we compare their respective objects to one another. For the object of the intellect is more simple and more absolute than the object of the will; since the object of the intellect is the very idea of appetible good; and the appetible good, the idea of which is in the intellect, is the object of the will. Now the more simple and the more abstract a thing is, the nobler and higher it is in itself; and therefore the object of the intellect is higher than the object of the will. Therefore, since the proper nature of a power is in its order to its object, it follows that the intellect in itself and absolutely is higher and nobler than the will. But relatively and by comparison with something else, we find that the will is sometimes higher than the intellect, from the fact that the object of the will occurs in something higher than that in which occurs the object of the intellect. Thus, for instance, I might say that hearing is relatively nobler than sight, inasmuch as something in which there is sound is nobler than something in which there is color, though color is nobler and simpler than sound. For as we have said above (Question 16, Article 1; Question 27, Article 4), the action of the intellect consists in this--that the idea of the thing understood is in the one who understands; while the act of the will consists in this--that the will is inclined to the thing itself as existing in itself. And therefore the Philosopher says in Metaph. vi (Did. v, 2) that "good and evil," which are objects of the will, "are in things," but "truth and error," which are objects of the intellect, "are in the mind." When, therefore, the thing in which there is good is nobler than the soul itself, in which is the idea understood; by comparison with such a thing, the will is higher than the intellect. But when the thing which is good is less noble than the soul, then even in comparison with that thing the intellect is higher than the will. Wherefore the love of God is better than the knowledge of God; but, on the contrary, the knowledge of corporeal things is better than the love thereof. Absolutely, however, the intellect is nobler than the will.
Reply to Objection 1. The aspect of causality is perceived by comparing one thing to another, and in such a comparison the idea of good is found to be nobler: but truth signifies something more absolute, and extends to the idea of good itself: wherefore even good is something true. But, again, truth is something good: forasmuch as the intellect is a thing, and truth its end. And among other ends this is the most excellent: as also is the intellect among the other powers.
Reply to Objection 2. What precedes in order of generation and time is less perfect: for in one and in the same thing potentiality precedes act, and imperfection precedes perfection. But what precedes absolutely and in the order of nature is more perfect: for thus act precedes potentiality. And in this way the intellect precedes the will, as the motive power precedes the thing movable, and as the active precedes the passive; for good which is understood moves the will.
Reply to Objection 3. This reason is verified of the will as compared with what is above the soul. For charity is the virtue by which we love God.
Source: Summa Theologica (New Advent)