Objection 1. It would seem that prophecy does not pertain to knowledge. For it is written (Sirach 48:14) that after death the body of Eliseus prophesied, and further on (Sirach 49:18) it is said of Joseph that "his bones were visited, and after death they prophesied." Now no knowledge remains in the body or in the bones after death. Therefore prophecy does not pertain to knowledge.
Objection 2. Further, it is written (1 Corinthians 14:3): "He that prophesieth, speaketh to men unto edification." Now speech is not knowledge itself, but its effect. Therefore it would seem that prophecy does not pertain to knowledge.
Objection 3. Further, every cognitive perfection excludes folly and madness. Yet both of these are consistent with prophecy; for it is written (Hosea 9:7): "Know ye, O Israel, that the prophet was foolish and mad [Vulgate: 'the spiritual man was mad']." Therefore prophecy is not a cognitive perfection.
Objection 4. Further, just as revelation regards the intellect, so inspiration regards, apparently, the affections, since it denotes a kind of motion. Now prophecy is described as "inspiration" or "revelation," according to Cassiodorus [Prolog. super Psalt. i]. Therefore it would seem that prophecy does not pertain to the intellect more than to the affections.
On the contrary, It is written (1 Samuel 9:9): "For he that is now called a prophet, in time past was called a seer." Now sight pertains to knowledge. Therefore prophecy pertains to knowledge.
I answer that, Prophecy first and chiefly consists in knowledge, because, to wit, prophets know things that are far [procul] removed from man's knowledge. Wherefore they may be said to take their name from phanos, "apparition," because things appear to them from afar. Wherefore, as Isidore states (Etym. vii, 8), "in the Old Testament, they were called Seers, because they saw what others saw not, and surveyed things hidden in mystery." Hence among heathen nations they were known as "vates, on account of their power of mind [vi mentis]," [The Latin 'vates' is from the Greek phates, and may be rendered 'soothsayer'] (Etym. viii, 7).
Since, however, it is written (1 Corinthians 12:7): "The manifestation of the Spirit is given to every man unto profit," and further on (1 Corinthians 14:12): "Seek to abound unto the edification of the Church," it follows that prophecy consists secondarily in speech, in so far as the prophets declare for the instruction of others, the things they know through being taught of God, according to the saying of Isaiah 21:10, "That which I have heard of the Lord of hosts, the God of Israel, I have declared unto you." Accordingly, as Isidore says (Etym. viii, 7), "prophets" may be described as "proefatores [foretellers], because they tell from afar [porro fantur]," that is, speak from a distance, "and foretell the truth about things to come."
Now those things above human ken which are revealed by God cannot be confirmed by human reason, which they surpass as regards the operation of the Divine power, according to Mark 16:20, "They... preached everywhere, the Lord working withal and confirming the word with signs that followed." Hence, thirdly, prophecy is concerned with the working of miracles, as a kind of confirmation of the prophetic utterances. Wherefore it is written (Deuteronomy 34:10-11): "There arose no more a prophet in Israel like unto Moses, whom the Lord knew face to face, in all the signs and wonders."
Reply to Objection 1. These passages speak of prophecy in reference to the third point just mentioned, which regards the proof of prophecy.
Reply to Objection 2. The Apostle is speaking there of the prophetic utterances.
Reply to Objection 3. Those prophets who are described as foolish and mad are not true but false prophets, of whom it is said (Jeremiah 3:16): "Hearken not to the words of the prophets that prophesy to you, and deceive you; they speak a vision of their own heart, and not out of the mouth of the Lord," and (Ezekiel 13:3): "Woe to the foolish prophets, that follow their own spirit, and see nothing."
Reply to Objection 4. It is requisite to prophecy that the intention of the mind be raised to the perception of Divine things: wherefore it is written (Ezekiel 2:1): "Son of man, stand upon thy feet, and I will speak to thee." This raising of the intention is brought about by the motion of the Holy Ghost, wherefore the text goes on to say: "And the Spirit entered into me... and He set me upon my feet." After the mind's intention has been raised to heavenly things, it perceives the things of God; hence the text continues: "And I heard Him speaking to me." Accordingly inspiration is requisite for prophecy, as regards the raising of the mind, according to Job 32:8, "The inspiration of the Almighty giveth understanding": while revelation is necessary, as regards the very perception of Divine things, whereby prophecy is completed; by its means the veil of darkness and ignorance is removed, according to Job 12:22, "He discovereth great things out of darkness."
Source: Summa Theologica (New Advent)