I answer that, One angel enlightens another. To make this clear, we must observe that intellectual light is nothing else than a manifestation of truth, according to Ephesians 5:13: "All that is made manifest is light." Hence to enlighten means nothing else but to communicate to others the manifestation of the known truth; according to the Apostle (Ephesians 3:8): "To me the least of all the saints is given this grace . . . to enlighten all men, that they may see what is the dispensation of the mystery which hath been hidden from eternity in God." Therefore one angel is said to enlighten another by manifesting the truth which he knows himself. Hence Dionysius says (Coel. Hier. vii): "Theologians plainly show that the orders of the heavenly beings are taught Divine science by the higher minds."
Now since two things concur in the intellectual operation, as we have said (105, 3), namely, the intellectual power, and the likeness of the thing understood; in both of these one angel can notify the known truth to another.
First, by strengthening his intellectual power; for just as the power of an imperfect body is strengthened by the neighborhood of a more perfect body --for instance, the less hot is made hotter by the presence of what is hotter; so the intellectual power of an inferior angel is strengthened by the superior angel turning to him: since in spiritual things, for one thing to turn to another, corresponds to neighborhood in corporeal things.
Secondly, one angel manifests the truth to another as regards the likeness of the thing understood. For the superior angel receives the knowledge of truth by a kind of universal conception, to receive which the inferior angel's intellect is not sufficiently powerful, for it is natural to him to receive truth in a more particular manner. Therefore the superior angel distinguishes, in a way, the truth which he conceives universally, so that it can be grasped by the inferior angel; and thus he proposes it to his knowledge. Thus it is with us that the teacher, in order to adapt himself to others, divides into many points the knowledge which he possesses in the universal. This is thus expressed by Dionysius (Coel. Hier. xv): "Every intellectual substance with provident power divides and multiplies the uniform knowledge bestowed on it by one nearer to God, so as to lead its inferiors upwards by analogy."