Intellect is said to be in potentia in two ways: one, before it possesses the habit of knowledge; the other, after the habit, but without actual consideration. According to the former, the angels are never in potentia in respect of natural knowledge, their mind being always complete through the species which form part of their nature. In things which pertain to Divine Revelation, however, there is nothing to prevent their being in potentia.
In the second manner they may be in potentia to things which they know by nature, since every object of their intelligence is not always present by actual consideration; that, however, which they see in the Word they possess always actually; for in this consists their Beatitude, which is always actual.
And in the Word they know everything simultaneously, because such knowledge is by means of one species, namely, the Divine Essence; and the object of a single operation is one, as there is one end of one motion. Therefore, what they know through the Word they know at once; and by natural knowledge they also know simultaneously whatever is represented by one species; but things which are known by diverse species they cannot know simultaneously.
As celestial bodies differ from terrestrial ones in having their perfections by nature, without movement or change, the angels possess by nature a complete and continual contemplation of truth without the discursive reasons indispensable to us. Hence the angels are called intellectual; and our souls are called rational.
If our intellect, indeed, saw from the beginning the force of a conclusion it would not need to understand discursively; nor if we knew always by direct apprehension of a subject what may be attributed to or subtracted from it, should we ever have to make use of composition and division; but because owing to the weakness of our intellect we cannot understand by a first apprehension, we reason, affirming and denying; a process of which the angels have no need, on account of their perfect intellectual light. Hence the angels do not understand by reasoning; nevertheless they understand the value of propositions and the reasoning of syllogisms.
Nor can there be fallacy in the angelic intellect, for the intellect is always true in its first operation. In us deception and fallacy supervene by accident; as when we reason about anything by means of some unsuitable composition, or mistake the definition of one thing for that of another. But this cannot happen with the angels, who do not make use of a process of reasoning, but know by a simple apprehension the definitions pertaining to each object. It is evident, moreover, that the essence of things may be a principle of knowledge in their regard as to what is naturally suitable or unsuitable to them, although not in those things which depend upon the supernatural ordination of God. Therefore the good angels, who have a right will, when they judge of supernatural things, - saving the ordinance of God, - cannot err; but the demons who have fallen from Divine Wisdom through a perverse will are often deceived in supernatural things, though not in natural ones.
According to Saint Augustine, there is a twofold knowledge in the angels: one of the mornings as it is called, which regards things in the Word; the other of the evening, as they know by their own nature; not through species taken from the things themselves, but by their own innate species; and although one is more perfect than the other, it need not exclude the other; for two operations may coexist in the same faculty, as the will desires at once the end, and the means to the end; or the intellect understands at the same time conditions and principles, by means of those same principles which enabled it to acquire the knowledge.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni