Commentaria in Scripturam Sacram, Vol. 11: Prolegomena (prefaces and canons)
This is what St. Gregory says, bk. iv Dialogues, ch. xlviii: “Holy men amid illusions and revelations discern the very voices or images of the visions by a certain inner savor, that they may know either what they perceive from the good spirit, or what they suffer from the deceiver.” So St. Augustine reports of his mother in the Confessions, that she was wont by a certain spiritual taste to discern and distinguish the inspirations and revelations of God from the suggestions of nature or of demons. For this reason the Prophets were bound by divine faith to believe their oracles, or whatever they heard from God, as revealed by the first Truth. For these are the proper and adequate material object of faith, whose formal object is divine revelation itself, or the first Truth revealing: for God so certainly demonstrated to the Prophets both the prophecies revealed by himself and that he was the one revealing them, that they believed both by divine faith: otherwise they would have exposed both themselves and God to the danger of falsehood and mockery; which, besides infamy, would have been a huge crime of sacrilege. But when the Prophets spoke only by some instinct (which, as St. Augustine says, human minds sometimes suffer even unawares), they were not plainly certain that it was from God: hence then they did not dare to propose it to the people as an oracle of God. Moreover it is now certain by the declaration of the Church that these our Prophets spoke from God all that they write in these books. For these are the prophets and canonical books of Scripture.
(St. Gregory, Homily 4 on Ezekiel, notes this carefully: “It must be known,” he says, “that the holy Prophets, when they are consulted, sometimes, from long practice of prophesying, bring forth certain things from their own spirit, and suppose that they are speaking them from prophecy; but because they are holy, they are quickly corrected by the Holy Spirit, hear from Him what is true, and rebuke themselves for what they have said falsely.”) So Nathan approved David’s plan of building the temple; but soon, taught the contrary by God, he retracted his word, 2 Kings 7:5. And in this the true Prophets are distinguished from the false: “For the false,” says St. Gregory, “both announce falsehoods, and, estranged from the Holy Spirit, persevere in their falsehood.” If therefore the Prophets attend carefully, they easily perceive whether God is speaking in them, or a demon, or reason, or their own desire: for God makes Himself manifest by sure signs, and makes them certain of what they hear from God. This is what St. Peter says, 2 Peter 1, last verse: “For prophecy came not by the will of man at any time: but the holy men of God spoke, inspired by the Holy Ghost.” See St. Thomas, II-II, Question 171, art. 5. For this reason the Prophets put the name of God at the beginning of their discourse: “Lest they should seem to speak from their own wit; but that what was said by them might be, as it were, divine oracles, and letters” brought down “from heaven,” says St. Chrysostom on Isaias, chapter 1, verse 1. And on this account prophecy is set apart from augury and necromancy, such as that by which the witch raised up Samuel for Saul, 1 Kings 28, and from every other divination which is drawn from the help of demons, or even from the natural sagacity of the Angels. For that a demon can naturally foreknow and foretell many things which man cannot, is taught by St. Augustine, in his book On the Divination of Demons, where he lists five ways in which demons naturally foretell the future.
For the same reason prophecy is set apart from the casting of lots for consultation, even lawful ones: “Lots,” says the Wise Man, Proverbs 16:33, “are cast into the lap, but they are tempered by the Lord”: tempered, I say, either by God’s ordinary governance and providence, as other things are; or by the extraordinary concourse and disposition of God intervening, as God appointed Matthias to be an Apostle by lot, Acts 1.
For the same reason prophecy is distinguished from conjecture, which is made by natural skill from signs, whether physiognomic or other natural ones, of which examples may also be seen in the sacred Scriptures: indeed, many rightly explain by chiromancy—not magical, but natural—that passage of Job 37:7: “Who sealeth up the hand of all men, that every one may know his works,” as I showed in that place. Some add to these astrological divination, from Judges 5:20, where it is said: “From heaven there was war against them: the stars remaining in their order and courses, fought against Sisara.” But this smacks of superstition and is an invention of the Rabbis. For that place in Judges 5 has a far different meaning, as I showed there. So St. Chrysostom on chapter 3 of Isaias, vol. 1: “The Prophet,” he says, “is distinguished from the conjecturer, as Eliseus from Solomon: the former by revelation, the latter by the course of reasoning, from signs and from experience, discovers and sees through hidden things. For so Solomon settled the quarrel of the two harlots over a child, putting both to the test, to see which loved the child more. But Eliseus knew Giezi’s avarice and his taking of gold by no sign, by no reasoning, but by God’s revelation alone.”
I said secondly, in the definition of prophecy, that it is an act of “future contingents,” that is, of free acts, or of things dependent on them, namely, things which will come to be by the free will of man or angel, on which they depend either immediately or mediately, so that they proceed freely from it and in their own time come into act.
For these alone are properly contingent. For the other natural things which the physicists call contingent, such as future rains, heat, cold, storms, etc., since they depend on natural causes, namely vapor, dryness, winds, etc., which are determinate, are therefore themselves determinate and necessary, and can be foreknown and foreseen by Angels and demons, who are most skilled in natural causes, in physics and astronomy, unless the free will of someone intervenes and alters and diminishes the combination and course of the secondary causes. Only free future acts, then, are contingent, because they are undetermined until the free choice freely determines itself to this part or that, as it will freely determine itself in future time, when it shall exist and act: these therefore neither angel nor demon can foreknow; God alone can, who by His infinite power of understanding penetrates and comprehends the liberty of all creatures, and sees from the immensity of His intelligence, clearly and distinctly, what that liberty will freely determine, do, and not do at any time, place and circumstance.
I said thirdly: “With certain and immovable truth”: because some restrict this to prophecy of foreknowledge, or absolute, so as to exclude prophecy of threat or of promise, or conditional: for this, they think, is not of immovable truth, because it is not always fulfilled. Here note: St. Thomas, II-II, Question 174, art. 1, lays down a threefold prophecy: the first of predestination, by which God foreknows and announces what He Himself has decreed to do and has predestined, and which therefore will come to pass most certainly. The second of foreknowledge, by which He foreknows good and evil acts to come, as much of man as of angel. The third of threat, by which He threatens, for example, the Ninivites with destruction through Jonas. The first two are certainly of immovable and absolute truth; the third is not: for it is plain that the Ninivites escaped by penance the destruction threatened to them; and so this prophecy and threat was not in fact fulfilled. But it is more correctly said that the first two kinds of prophecy are absolute, and therefore have absolute truth; the third, however, since it is conditional, also has its own truth, but a conditional one, commensurate with itself: for it cannot have an absolute truth, since it is not itself absolute, but conditional. For the threat made by Jonas to the Ninivites was this: Unless you do penance, you shall be overthrown. This proposition was conditional, and true conditionally; but because the Ninivites by doing penance removed this condition, they removed also the destruction attached to it, and brought it about that what God threatened them with would not absolutely come to pass; because they performed what God intended by this threat, namely penance and a change of morals, that by it they might escape the ruin threatened by Him, as by doing penance they in fact escaped it.