Objection 1. It would seem that the knowledge of an angel is discursive. For the discursive movement of the mind comes from one thing being known through another. But the angels know one thing through another; for they know creatures through the Word. Therefore the intellect of an angel knows by discursive method.
Objection 2. Further, whatever a lower power can do, the higher can do. But the human intellect can syllogize, and know causes in effects; all of which is the discursive method. Therefore the intellect of the angel, which is higher in the order of nature, can with greater reason do this.
Objection 3. Further, Isidore (De sum. bono i, 10) says that "demons learn more things by experience." But experimental knowledge is discursive: for, "one experience comes of many remembrances, and one universal from many experiences," as Aristotle observes (Poster. ii; Metaph. vii). Therefore an angel's knowledge is discursive.
On the contrary, Dionysius says (Div. Nom. vii) that the "angels do not acquire Divine knowledge from separate discourses, nor are they led to something particular from something common."
I answer that, As has often been stated (1; 55, 1), the angels hold that grade among spiritual substances which the heavenly bodies hold among corporeal substances: for Dionysius calls them "heavenly minds" (1; 55, 1). Now, the difference between heavenly and earthly bodies is this, that earthly bodies obtain their last perfection by chance and movement: while the heavenly bodies have their last perfection at once from their very nature. So, likewise, the lower, namely, the human, intellects obtain their perfection in the knowledge of truth by a kind of movement and discursive intellectual operation; that is to say, as they advance from one known thing to another. But, if from the knowledge of a known principle they were straightway to perceive as known all its consequent conclusions, then there would be no discursive process at all. Such is the condition of the angels, because in the truths which they know naturally, they at once behold all things whatsoever that can be known in them.
Therefore they are called "intellectual beings": because even with ourselves the things which are instantly grasped by the mind are said to be understood [intelligi]; hence "intellect" is defined as the habit of first principles. But human souls which acquire knowledge of truth by the discursive method are called "rational"; and this comes of the feebleness of their intellectual light. For if they possessed the fulness of intellectual light, like the angels, then in the first aspect of principles they would at once comprehend their whole range, by perceiving whatever could be reasoned out from them.
Reply to Objection 1. Discursion expresses movement of a kind. Now all movement is from something before to something after. Hence discursive knowledge comes about according as from something previously known one attains to the knowledge of what is afterwards known, and which was previously unknown. But if in the thing perceived something else be seen at the same time, as an object and its image are seen simultaneously in a mirror, it is not discursive knowledge. And in this way the angels know things in the Word.
Reply to Objection 2. The angels can syllogize, in the sense of knowing a syllogism; and they see effects in causes, and causes in effects: yet they do not acquire knowledge of an unknown truth in this way, by syllogizing from causes to effect, or from effect to cause.
Reply to Objection 3. Experience is affirmed of angels and demons simply by way of similitude, forasmuch as they know sensible things which are present, yet without any discursion withal.
Source: Summa Theologica (New Advent)