Objection 1. It would seem that the prophet always distinguishes what he says by his own spirit from what he says by the prophetic spirit. For Augustine states (Confess. vi, 13) that his mother said "she could, through a certain feeling, which in words she could not express, discern betwixt Divine revelations, and the dreams of her own soul." Now prophecy is a Divine revelation, as stated above (Article 3). Therefore the prophet always distinguishes what he says by the spirit of prophecy, from what he says by his own spirit.
Objection 2. Further, God commands nothing impossible, as Jerome [Pelagius, Ep. xvi, among the supposititious works of St. Jerome] says. Now the prophets were commanded (Jeremiah 23:28): "The prophet that hath a dream, let him tell a dream; and he that hath My word, let him speak My word with truth." Therefore the prophet can distinguish what he has through the spirit of prophecy from what he sees otherwise.
Objection 3. Further, the certitude resulting from a Divine light is greater than that which results from the light of natural reason. Now he that has science, by the light of natural reason knows for certain that he has it. Therefore he that has prophecy by a Divine light is much more certain that he has it.
On the contrary, Gregory says (Hom. i super Ezech.): "It must be observed that sometimes the holy prophets, when consulted, utter certain things by their own spirit, through being much accustomed to prophesying, and think they are speaking by the prophetic spirit."
I answer that, The prophet's mind is instructed by God in two ways: in one way by an express revelation, in another way by a most mysterious instinct to "which the human mind is subjected without knowing it," as Augustine says (Gen. ad lit. ii, 17). Accordingly the prophet has the greatest certitude about those things which he knows by an express revelation, and he has it for certain that they are revealed to him by God; wherefore it is written (Jeremiah 26:15): "In truth the Lord sent me to you, to speak all these words in your hearing." Else, were he not certain about this, the faith which relies on the utterances of the prophet would not be certain. A sign of the prophet's certitude may be gathered from the fact that Abraham being admonished in a prophetic vision, prepared to sacrifice his only-begotten son, which he nowise would have done had he not been most certain of the Divine revelation.
On the other hand, his position with regard to the things he knows by instinct is sometimes such that he is unable to distinguish fully whether his thoughts are conceived of Divine instinct or of his own spirit. And those things which we know by Divine instinct are not all manifested with prophetic certitude, for this instinct is something imperfect in the genus of prophecy. It is thus that we are to understand the saying of Gregory. Lest, however, this should lead to error, "they are very soon set aright by the Holy Ghost [For instance, cf. 2 Samuel 7:3 seqq.], and from Him they hear the truth, so that they reproach themselves for having said what was untrue," as Gregory adds (Hom. i super Ezech.).
The arguments set down in the first place consider the revelation that is made by the prophetic spirit; wherefore the answer to all the objections is clear.
Source: Summa Theologica (New Advent)