Objection 1. It would seem that the angels understand by composing and dividing. For, where there is multiplicity of things understood, there is composition of the same, as is said in De Anima iii, text. 21. But there is a multitude of things understood in the angelic mind; because angels apprehend different things by various species, and not all at one time. Therefore there is composition and division in the angel's mind.
Objection 2. Further, negation is far more remote from affirmation than any two opposite natures are; because the first of distinctions is that of affirmation and negation. But the angel knows certain distant natures not by one, but by diverse species, as is evident from what was said (2). Therefore he must know affirmation and negation by diverse species. And so it seems that he understands by composing and dividing.
Objection 3. Further, speech is a sign of the intellect. But in speaking to men, angels use affirmative and negative expressions, which are signs of composition and of division in the intellect; as is manifest from many passages of Sacred Scripture. Therefore it seems that the angel understands by composing and dividing.
On the contrary, Dionysius says (Div. Nom. vii) that "the intellectual power of the angel shines forth with the clear simplicity of divine concepts." But a simple intelligence is without composition and division. Therefore the angel understands without composition or division.
I answer that, As in the intellect, when reasoning, the conclusion is compared with the principle, so in the intellect composing and dividing, the predicate is compared with the subject. For if our intellect were to see at once the truth of the conclusion in the principle, it would never understand by discursion and reasoning. In like manner, if the intellect in apprehending the quiddity of the subject were at once to have knowledge of all that can be attributed to, or removed from, the subject, it would never understand by composing and dividing, but only by understanding the essence. Thus it is evident that for the self-same reason our intellect understands by discursion, and by composing and dividing, namely, that in the first apprehension of anything newly apprehended it does not at once grasp all that is virtually contained in it. And this comes from the weakness of the intellectual light within us, as has been said (3). Hence, since the intellectual light is perfect in the angel, for he is a pure and most clear mirror, as Dionysius says (Div. Nom. iv), it follows that as the angel does not understand by reasoning, so neither does he by composing and dividing.
Nevertheless, he understands the composition and the division of enunciations, just as he apprehends the reasoning of syllogisms: for he understands simply, such things as are composite, things movable immovably, and material things immaterially.
Reply to Objection 1. Not every multitude of things understood causes composition, but a multitude of such things understood that one of them is attributed to, or denied of, another. When an angel apprehends the nature of anything, he at the same time understands whatever can be either attributed to it, or denied of it. Hence, in apprehending a nature, he by one simple perception grasps all that we can learn by composing and dividing.
Reply to Objection 2. The various natures of things differ less as to their mode of existing than do affirmation and negation. Yet, as to the way in which they are known, affirmation and negation have something more in common; because directly the truth of an affirmation is known, the falsehood of the opposite negation is known also.
Reply to Objection 3. The fact that angels use affirmative and negative forms of speech, shows that they know both composition and division: yet not that they know by composing and dividing, but by knowing simply the nature of a thing.
Source: Summa Theologica (New Advent)