Objection 1. It would seem that the degrees of prophecy cannot be distinguished according to the imaginary vision. For the degrees of a thing bear relation to something that is on its own account, not on account of something else. Now, in prophecy, intellectual vision is sought on its own account, and imaginary vision on account of something else, as stated above (2, ad 2). Therefore it would seem that the degrees of prophecy are distinguished not according to imaginary, but only according to intellectual, vision.
Objection 2. Further, seemingly for one prophet there is one degree of prophecy. Now one prophet receives revelation through various imaginary visions. Therefore a difference of imaginary visions does not entail a difference of prophecy.
Objection 3. Further, according to a gloss [Cassiodorus, super Prolog. Hieron. in Psalt.], prophecy consists of words, deeds, dreams, and visions. Therefore the degrees of prophecy should not be distinguished according to imaginary vision, to which vision and dreams pertain, rather than according to words and deeds.
On the contrary, The medium differentiates the degrees of knowledge: thus science based on direct ["Propter quid"] proofs is more excellent than science based on indirect ["Quia"] premises or than opinion, because it comes through a more excellent medium. Now imaginary vision is a kind of medium in prophetic knowledge. Therefore the degrees of prophecy should be distinguished according to imaginary vision.
I answer that, As stated above (Question 173, Article 2), the prophecy wherein, by the intelligible light, a supernatural truth is revealed through an imaginary vision, holds the mean between the prophecy wherein a supernatural truth is revealed without imaginary vision, and that wherein through the intelligible light and without an imaginary vision, man is directed to know or do things pertaining to human conduct. Now knowledge is more proper to prophecy than is action; wherefore the lowest degree of prophecy is when a man, by an inward instinct, is moved to perform some outward action. Thus it is related of Samson (Judges 15:14) that "the Spirit of the Lord came strongly upon him, and as the flax ['Lina.' St. Thomas apparently read 'ligna' ('wood')] is wont to be consumed at the approach of fire, so the bands with which he was bound were broken and loosed." The second degree of prophecy is when a man is enlightened by an inward light so as to know certain things, which, however, do not go beyond the bounds of natural knowledge: thus it is related of Solomon (1 Kings 4:32-33) that "he spoke... parables... and he treated about trees from the cedar that is in Libanus unto the hyssop that cometh out of the wall, and he discoursed of beasts and of fowls, and of creeping things and of fishes": and all of this came from divine inspiration, for it was stated previously (1 Kings 4:29): "God gave to Solomon wisdom and understanding exceeding much."
Nevertheless these two degrees are beneath prophecy properly so called, because they do not attain to supernatural truth. The prophecy wherein supernatural truth is manifested through imaginary vision is differentiated first according to the difference between dreams which occur during sleep, and vision which occurs while one is awake. The latter belongs to a higher degree of prophecy, since the prophetic light that draws the soul away to supernatural things while it is awake and occupied with sensible things would seem to be stronger than that which finds a man's soul asleep and withdrawn from objects of sense. Secondly the degrees of this prophecy are differentiated according to the expressiveness of the imaginary signs whereby the intelligible truth is conveyed. And since words are the most expressive signs of intelligible truth, it would seem to be a higher degree of prophecy when the prophet, whether awake or asleep, hears words expressive of an intelligible truth, than when he sees things significative of truth, for instance "the seven full ears of corn" signified "seven years of plenty" (Genesis 41:22-26). On such like signs prophecy would seem to be the more excellent, according as the signs are more expressive, for instance when Jeremias saw the burning of the city under the figure of a boiling cauldron (Jeremiah 1:13). Thirdly, it is evidently a still higher degree of prophecy when a prophet not only sees signs of words or deeds, but also, either awake or asleep, sees someone speaking or showing something to him, since this proves the prophet's mind to have approached nearer to the cause of the revelation. Fourthly, the height of a degree of prophecy may be measured according to the appearance of the person seen: for it is a higher degree of prophecy, if he who speaks or shows something to the waking or sleeping prophet be seen by him under the form of an angel, than if he be seen by him under the form of man: and higher still is it, if he be seen by the prophet whether asleep or awake, under the appearance of God, according to Isaiah 6:1, "I saw the Lord sitting."
But above all these degrees there is a third kind of prophecy, wherein an intelligible and supernatural truth is shown without any imaginary vision. However, this goes beyond the bounds of prophecy properly so called, as stated above (2, ad 3); and consequently the degrees of prophecy are properly distinguished according to imaginary vision.
Reply to Objection 1. We are unable to know how to distinguish the intellectual light, except by means of imaginary or sensible signs. Hence the difference in the intellectual light is gathered from the difference in the things presented to the imagination.
Reply to Objection 2. As stated above (Question 171, Article 2), prophecy is by way, not of an abiding habit, but of a transitory passion; wherefore there is nothing inconsistent if one and the same prophet, at different times, receive various degrees of prophetic revelation.
Reply to Objection 3. The words and deeds mentioned there do not pertain to the prophetic revelation, but to the announcement, which is made according to the disposition of those to whom that which is revealed to the prophet is announced; and this is done sometimes by words, sometimes by deeds. Now this announcement, and the working of miracles, are something consequent upon prophecy, as stated above (Question 171, Article 1).
Source: Summa Theologica (New Advent)