Thomas Aquinas, Compendium of Theology

The substances mentioned above, which are called immaterial, must also be intellectual. A being is intellectual for the reason that it is free from matter. This can be perceived from the very way it understands. The intelligible in act and the intellect in act are the same thing. But it is clear that a thing is intelligible in act because it is separated from matter; we cannot have intellectual knowledge of material things except by abstracting from matter. Accordingly we must pronounce the same judgment regarding the intellect; that is, whatever is immaterial, is intellectual.

Furthermore, immaterial substances hold the first place and are supreme among beings; for act naturally has precedence over potency. But the intellect is clearly superior to all other beings; for the intellect uses corporeal things as instruments. Therefore immaterial substances must be intellectual.

Moreover, the higher a thing is in the scale of being, the closer it draws to likeness with God. Thus we observe that some things, those pertaining to the lowest degree, such as lifeless beings, share in the divine likeness with respect to existence only; others, for example, plants, share in the divine likeness with respect to existence and life; yet others, such as animals, with respect to sense perception. But the highest degree, and that which makes us most like to God, is conferred by the intellect. Consequently the most excellent creatures are intellectual. Indeed, they are said to be fashioned in God's image for the very reason that among all creatures they approach most closely to likeness with God.