Objection 1. It would seem that the morning and the evening knowledge are one. For it is said (Genesis 1:5): "There was evening and morning, one day." But by the expression "day" the knowledge of the angels is to be understood, as Augustine says (Gen. ad lit. iv, 23). Therefore the morning and evening knowledge of the angels are one and the same.
Objection 2. Further, it is impossible for one faculty to have two operations at the same time. But the angels are always using their morning knowledge; because they are always beholding God and things in God, according to Matthew 18:10. Therefore, if the evening knowledge were different from the morning, the angel could never exercise his evening knowledge.
Objection 3. Further, the Apostle says (1 Corinthians 13:10): "When that which is perfect is come, then that which is in part shall be done away." But, if the evening knowledge be different from the morning, it is compared to it as the less perfect to the perfect. Therefore the evening knowledge cannot exist together with the morning knowledge.
On the contrary, Augustine says (Gen. ad lit. iv, 24): "There is a vast difference between knowing anything as it is in the Word of God, and as it is in its own nature; so that the former belongs to the day, and the latter to the evening."
I answer that, As was observed (6), the evening knowledge is that by which the angels know things in their proper nature. This cannot be understood as if they drew their knowledge from the proper nature of things, so that the preposition "in" denotes the form of a principle; because, as has been already stated (55, 2), the angels do not draw their knowledge from things. It follows, then, that when we say "in their proper nature" we refer to the aspect of the thing known in so far as it is an object of knowledge; that is to say, that the evening knowledge is in the angels in so far as they know the being of things which those things have in their own nature.
Now they know this through a twofold medium, namely, by innate ideas, or by the forms of things existing in the Word. For by beholding the Word, they know not merely the being of things as existing in the Word, but the being as possessed by the things themselves; as God by contemplating Himself sees that being which things have in their own nature. If, therefore, it be called evening knowledge, in so far as when the angels behold the Word, they know the being which things have in their proper nature, then the morning and the evening knowledge are essentially one and the same, and only differ as to the things known. If it be called evening knowledge, in so far as through innate ideas they know the being which things have in their own natures, then the morning and the evening knowledge differ. Thus Augustine seems to understand it when he assigns one as inferior to the other.
Reply to Objection 1. The six days, as Augustine understands them, are taken as the six classes of things known by the angels; so that the day's unit is taken according to the unit of the thing understood; which, nevertheless, can be apprehended by various ways of knowing it.
Reply to Objection 2. There can be two operations of the same faculty at the one time, one of which is referred to the other; as is evident when the will at the same time wills the end and the means to the end; and the intellect at the same instant perceives principles and conclusions through those principles, when it has already acquired knowledge. As Augustine says (Gen. ad lit. iv, 24), the evening knowledge is referred to the morning knowledge in the angels; hence there is nothing to hinder both from being at the same time in the angels.
Reply to Objection 3. On the coming of what is perfect, the opposite imperfect is done away: just as faith, which is of the things that are not seen, is made void when vision succeeds. But the imperfection of the evening knowledge is not opposed to the perfection of the morning knowledge. For that a thing be known in itself, is not opposite to its being known in its cause. Nor, again, is there any inconsistency in knowing a thing through two mediums, one of which is more perfect and the other less perfect; just as we can have a demonstrative and a probable medium for reaching the same conclusion. In like manner a thing can be known by the angel through the uncreated Word, and through an innate idea.
Source: Summa Theologica (New Advent)