Objection 1. It would seem that the angels know secret thoughts. For Gregory (Moral. xviii), explaining Job 28:17: "Gold or crystal cannot equal it," says that "then," namely in the bliss of those rising from the dead, "one shall be as evident to another as he is to himself, and when once the mind of each is seen, his conscience will at the same time be penetrated." But those who rise shall be like the angels, as is stated (Matthew 22:30). Therefore an angel can see what is in another's conscience.
Objection 2. Further, intelligible species bear the same relation to the intellect as shapes do to bodies. But when the body is seen its shape is seen. Therefore, when an intellectual substance is seen, the intelligible species within it is also seen. Consequently, when one angel beholds another, or even a soul, it seems that he can see the thoughts of both.
Objection 3. Further, the ideas of our intellect resemble the angel more than do the images in our imagination; because the former are actually understood, while the latter are understood only potentially. But the images in our imagination can be known by an angel as corporeal things are known: because the imagination is a corporeal faculty. Therefore it seems that an angel can know the thoughts of the intellect.
On the contrary, What is proper to God does not belong to the angels. But it is proper to God to read the secrets of hearts, according to Jeremiah 17:9: "The heart is perverse above all things, and unsearchable; who can know it? I am the Lord, Who search the heart." Therefore angels do not know the secrets of hearts.
I answer that, A secret thought can be known in two ways: first, in its effect. In this way it can be known not only by an angel, but also by man; and with so much the greater subtlety according as the effect is the more hidden. For thought is sometimes discovered not merely by outward act, but also by change of countenance; and doctors can tell some passions of the soul by the mere pulse. Much more then can angels, or even demons, the more deeply they penetrate those occult bodily modifications. Hence Augustine says (De divin. daemon.) that demons "sometimes with the greatest faculty learn man's dispositions, not only when expressed by speech, but even when conceived in thought, when the soul expresses them by certain signs in the body"; although (Retract. ii, 30) he says "it cannot be asserted how this is done."
In another way thoughts can be known as they are in the mind, and affections as they are in the will: and thus God alone can know the thoughts of hearts and affections of wills. The reason of this is, because the rational creature is subject to God only, and He alone can work in it Who is its principal object and last end: this will be developed later (63, 1; 105, 5). Consequently all that is in the will, and all things that depend only on the will, are known to God alone. Now it is evident that it depends entirely on the will for anyone actually to consider anything; because a man who has a habit of knowledge, or any intelligible species, uses them at will. Hence the Apostle says (1 Corinthians 2:11): "For what man knoweth the things of a man, but the spirit of a man that is in him?"
Reply to Objection 1. In the present life one man's thought is not known by another owing to a twofold hindrance; namely, on account of the grossness of the body, and because the will shuts up its secrets. The first obstacle will be removed at the Resurrection, and does not exist at all in the angels; while the second will remain, and is in the angels now. Nevertheless the brightness of the body will show forth the quality of the soul; as to its amount of grace and of glory. In this way one will be able to see the mind of another.
Reply to Objection 2. Although one angel sees the intelligible species of another, by the fact that the species are proportioned to the rank of these substances according to greater or lesser universality, yet it does not follow that one knows how far another makes use of them by actual consideration.
Reply to Objection 3. The appetite of the brute does not control its act, but follows the impression of some other corporeal or spiritual cause. Since, therefore, the angels know corporeal things and their dispositions, they can thereby know what is passing in the appetite or in the imaginative apprehension of the brute beasts, and even of man, in so far as the sensitive appetite sometimes, through following some bodily impression, influences his conduct, as always happens in brutes. Yet the angels do not necessarily know the movement of the sensitive appetite and the imaginative apprehension of man in so far as these are moved by the will and reason; because, even the lower part of the soul has some share of reason, as obeying its ruler, as is said in Ethics iii, 12. But it does not follow that, if the angel knows what is passing through man's sensitive appetite or imagination, he knows what is in the thought or will: because the intellect or will is not subject to the sensitive appetite or the imagination, but can make various uses of them.
Source: Summa Theologica (New Advent)