The angels understand themselves by their own substance. For as transitive actions are separate from, and immanent ones are united to, the agent, those species which are always actualized are known by their possessors without any change being required. And since the angels are immaterial, of some subsistent form, and, as such, intelligible actually, it follows that they know themselves by means of their form, which is their substance.
Every angel likewise understands others; for as God creates things, not only in physical being but also as they are intelligible in the minds of the angels, there exist in every angel certain conceptions by which they understand both material and spiritual things. The conception of their own species, at once natural and intellectual, impressed upon angels individually, enables them to understand themselves; while the conceptions of other natures, whether spiritual or material, impressed according to intellectual being only, enable them to know creatures, both material and spiritual.
For things may be known in three ways: first, by the essence which exists in the knower; as an angel knows itself, or the eye sees the light: a mode of vision which bears a likeness to that Vision of God by His own Essence which surpasses the natural power of any created mind; for God is incomprehensible. Secondly, a thing is known by its own similitude; as we see a wall by species existing in the eye; and thus the angels know God in virtue of their nature, which, as a simple intellectual essence, bears a certain likeness to the Divine Nature. Thirdly, some things are known, not by their own species, but by species received from others; whence results some similitude to the thing known. In this way we see God in this present state, in visible things; which, according to the Apostle, is to see Him in an enigma. The natural knowledge which the angels possess of God is midway between these three modes, but nearer to the last, inasmuch as the angelic nature, not being a perfect likeness of the Divine Essence, does not perfectly reflect It.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni