Objection 1. It would seem that the prophets always know the things which they prophesy. For, as Augustine says (Gen. ad lit. xii, 9), "those to whom signs were shown in spirit by means of the likenesses of bodily things, had not the gift of prophecy, unless the mind was brought into action, so that those signs were also understood by them." Now what is understood cannot be unknown. Therefore the prophet is not ignorant of what he prophesies.
Objection 2. Further, the light of prophecy surpasses the light of natural reason. Now one who possesses a science by his natural light, is not ignorant of his scientific acquirements. Therefore he who utters things by the prophetic light cannot ignore them.
Objection 3. Further, prophecy is directed for man's enlightenment; wherefore it is written (2 Peter 1:19): "We have the more firm prophetical word, whereunto you do well to attend, as to a light that shineth in a dark place." Now nothing can enlighten others unless it be lightsome in itself. Therefore it would seem that the prophet is first enlightened so as to know what he declares to others.
On the contrary, It is written (John 11:51): "And this he" (Caiphas) "spoke, not of himself, but being the High Priest of that year, he prophesied that Jesus should die for the nation," etc. Now Caiphas knew this not. Therefore not every prophet knows what he prophesies.
I answer that, In prophetic revelation the prophet's mind is moved by the Holy Ghost, as an instrument that is deficient in regard to the principal agent. Now the prophet's mind is moved not only to apprehend something, but also to speak or to do something; sometimes indeed to all these three together, sometimes to two, sometimes to one only, and in each case there may be a defect in the prophet's knowledge. For when the prophet's mind is moved to think or apprehend a thing, sometimes he is led merely to apprehend that thing, and sometimes he is further led to know that it is divinely revealed to him.
Again, sometimes the prophet's mind is moved to speak something, so that he understands what the Holy Ghost means by the words he utters; like David who said (2 Samuel 23:2): "The Spirit of the Lord hath spoken by me"; while, on the other hand, sometimes the person whose mind is moved to utter certain words knows not what the Holy Ghost means by them, as was the case with Caiphas (John 11:51).
Again, when the Holy Ghost moves a man's mind to do something, sometimes the latter understands the meaning of it, like Jeremias who hid his loin-cloth in the Euphrates (Jeremiah 13:1-11); while sometimes he does not understand it--thus the soldiers, who divided Christ's garments, understood not the meaning of what they did.
Accordingly, when a man knows that he is being moved by the Holy Ghost to think something, or signify something by word or deed, this belongs properly to prophecy; whereas when he is moved, without his knowing it, this is not perfect prophecy, but a prophetic instinct. Nevertheless it must be observed that since the prophet's mind is a defective instrument, as stated above, even true prophets know not all that the Holy Ghost means by the things they see, or speak, or even do.
And this suffices for the Replies to the Objections, since the arguments given at the beginning refer to true prophets whose minds are perfectly enlightened from above.
Source: Summa Theologica (New Advent)