Objection 1. It would seem that the intelligence is another power than the intellect. For we read in De Spiritu et Anima that "when we wish to rise from lower to higher things, first the sense comes to our aid, then imagination, then reason, then intellect, and afterwards intelligence." But imagination and sense are distinct powers. Therefore also intellect and intelligence are distinct.
Objection 2. Further, Boethius says (De Consol. v, 4) that "sense considers man in one way, imagination in another, reason in another, intelligence in another." But intellect is the same power as reason. Therefore, seemingly, intelligence is a distinct power from intellect, as reason is a distinct power from imagination or sense.
Objection 3. Further, "actions came before powers," as the Philosopher says (De Anima ii, 4). But intelligence is an act separate from others attributed to the intellect. For Damascene says (De Fide Orth. ii) that "the first movement is called intelligence; but that intelligence which is about a certain thing is called intention; that which remains and conforms the soul to that which is understood is called invention, and invention when it remains in the same man, examining and judging of itself, is called phronesis [that is, wisdom], and phronesis if dilated makes thought, that is, orderly internal speech; from which, they say, comes speech expressed by the tongue." Therefore it seems that intelligence is some special power.
On the contrary, The Philosopher says (De Anima iii, 6) that "intelligence is of indivisible things in which there is nothing false." But the knowledge of these things belongs to the intellect. Therefore intelligence is not another power than the intellect.
I answer that, This word "intelligence" properly signifies the intellect's very act, which is to understand. However, in some works translated from the Arabic, the separate substances which we call angels are called "intelligences," and perhaps for this reason, that such substances are always actually understanding. But in works translated from the Greek, they are called "intellects" or "minds." Thus intelligence is not distinct from intellect, as power is from power; but as act is from power. And such a division is recognized even by the philosophers. For sometimes they assign four intellects--namely, the "active" and "passive" intellects, the intellect "in habit," and the "actual" intellect. Of which four the active and passive intellects are different powers; just as in all things the active power is distinct from the passive. But three of these are distinct, as three states of the passive intellect, which is sometimes in potentiality only, and thus it is called passive; sometimes it is in the first act, which is knowledge, and thus it is called intellect in habit; and sometimes it is in the second act, which is to consider, and thus it is called intellect in act, or actual intellect.
Reply to Objection 1. If this authority is accepted, intelligence there means the act of the intellect. And thus it is divided against intellect as act against power.
Reply to Objection 2. Boethius takes intelligence as meaning that act of the intellect which transcends the act of the reason. Wherefore he also says that reason alone belongs to the human race, as intelligence alone belongs to God, for it belongs to God to understand all things without any investigation.
Reply to Objection 3. All those acts which Damascene enumerates belong to one power--namely, the intellectual power. For this power first of all only apprehends something; and this act is called "intelligence."
Secondly, it directs what it apprehends to the knowledge of something else, or to some operation; and this is called "intention." And when it goes on in search of what it "intends," it is called "invention." When, by reference to something known for certain, it examines what it has found, it is said to know or to be wise, which belongs to "phronesis" or "wisdom"; for "it belongs to the wise man to judge," as the Philosopher says (Metaph. i, 2). And when once it has obtained something for certain, as being fully examined, it thinks about the means of making it known to others; and this is the ordering of "interior speech," from which proceeds "external speech." For every difference of acts does not make the powers vary, but only what cannot be reduced to the one same principle, as we have said above (Question 78, Article 4).
Source: Summa Theologica (New Advent)