/summa/FP/FPVII/FP_Q56:
We now inquire into the knowledge of the angels with regard to the objects known by them. We shall treat of their knowledge, first, of immaterial things, secondly of things material. Under the first heading there are three points of inquiry:
(1) Does an angel know himself?
(2) Does one angel know another?
(3) Does the angel know God by his own natural principles?