One angel can give light to another; for to give light in intellectual things is to manifest truth; and this may be done by the simple presentation of an object, and by bringing light to bear upon it. And one angel illuminates another by means of its intellectual power; for as a less perfect body is strengthened by the neighbourhood of a more perfect one, so the inferior angel receives the increase of intellectual power through the influence of a superior one.
Moreover, the superior apprehends truth in a more universal manner, and communicates it under suitable distinctions, so as to be understood by the inferior; even as teachers among us, by multiplying examples, convey to those of weaker capacity what they themselves understand under a single form.
But one angel cannot move the will of another, for the will can only be moved in two ways, viz. either directly by the object, or by means of the faculty. By the object it cannot be, for only the Universal Good moves the will sufficiently, i.e. God, Who is the Beatifying Object. An angel may, however, use persuasion, by pointing out some created good ordained in God. Neither does one angel move the will of another by means of the faculty, for God only can change the will, Who has given the power of willing.
Neither do the inferior angels illuminate the superior ones, because one Order is related to another as cause to cause, the superiors being the illuminators and the inferiors the illuminated; and if there be any deviation from the order of second causes it is for the sake of man's salvation, a reason which does not exist among the angels; for the object of miraculous action is to promote the knowledge of God in man.
Superior angels enlighten the inferior ones about all whereof they have knowledge: for it is the nature of good to diffuse itself, and the more any creature participates in the Divine Goodness the more it diffuses its perfections. Nevertheless, the superior retains its superiority, and the more perfect knowledge which belongs to a higher order; as a master understands various branches of knowledge more perfectly than the pupil who learns from him.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni