Objection 1. It would seem that prophecy can be natural. For Gregory says (Dial. iv, 26) that "sometimes the mere strength of the soul is sufficiently cunning to foresee certain things": and Augustine says (Gen. ad lit. xii, 13) that the human soul, according as it is withdrawn from the sense of the body, is able to foresee the future [Cf. I, 86, 4, ad 2]. Now this pertains to prophecy. Therefore the soul can acquire prophecy naturally.
Objection 2. Further, the human soul's knowledge is more alert while one wakes than while one sleeps. Now some, during sleep, naturally foresee the future, as the Philosopher asserts (De Somn. et Vigil. [De Divinat. per Somn. ii, which is annexed to the work quoted]). Much more therefore can a man naturally foreknow the future.
Objection 3. Further, man, by his nature, is more perfect than dumb animals. Yet some dumb animals have foreknowledge of future things that concern them. Thus ants foreknow the coming rains, which is evident from their gathering grain into their nest before the rain commences; and in like manner fish foreknow a coming storm, as may be gathered from their movements in avoiding places exposed to storm. Much more therefore can men foreknow the future that concerns themselves, and of such things is prophecy. Therefore prophecy comes from nature.
Objection 4. Further, it is written (Proverbs 29:18): "When prophecy shall fail, the people shall be scattered abroad"; wherefore it is evident that prophecy is necessary for the stability of the human race. Now "nature does not fail in necessaries" [Aristotle, de Anima iii, 9. Therefore it seems that prophecy is from nature.
On the contrary, It is written (2 Peter 1:21): "For prophecy came not by the will of man at any time, but the holy men of God spoke, inspired by the Holy Ghost." Therefore prophecy comes not from nature, but through the gift of the Holy Ghost.
I answer that, As stated above (171, 6, ad 2) prophetic foreknowledge may regard future things in two ways: in one way, as they are in themselves; in another way, as they are in their causes. Now, to foreknow future things, as they are in themselves, is proper to the Divine intellect, to Whose eternity all things are present, as stated in I, 14, 13. Wherefore such like foreknowledge of the future cannot come from nature, but from Divine revelation alone. On the other hand, future things can be foreknown in their causes with a natural knowledge even by man: thus a physician foreknows future health or death in certain causes, through previous experimental knowledge of the order of those causes to such effects. Such like knowledge of the future may be understood to be in a man by nature in two ways. On one way that the soul, from that which it holds, is able to foreknow the future, and thus Augustine says (Gen. ad lit. xii, 13): "Some have deemed the human soul to contain a certain power of divination." This seems to be in accord with the opinion of Plato [Phaed. xxvii; Civit. vi], who held that our souls have knowledge of all things by participating in the ideas; but that this knowledge is obscured in them by union with the body; yet in some more, in others less, according to a difference in bodily purity. According to this it might be said that men, whose souls are not much obscured through union with the body, are able to foreknow such like future things by their own knowledge. Against this opinion Augustine says (Gen. ad lit. xii, 13): "How is it that the soul cannot always have this power of divination, since it always wishes to have it?"
Since, however, it seems truer, according to the opinion of Aristotle, that the soul acquires knowledge from sensibles, as stated in I, 84, 6, it is better to have recourse to another explanation, and to hold that men have no such foreknowledge of the future, but that they can acquire it by means of experience, wherein they are helped by their natural disposition, which depends on the perfection of a man's imaginative power, and the clarity of his understanding.
Nevertheless this latter foreknowledge of the future differs in two ways from the former, which comes through Divine revelation. First, because the former can be about any events whatever, and this infallibly; whereas the latter foreknowledge, which can be had naturally, is about certain effects, to which human experience may extend. Secondly, because the former prophecy is "according to the unchangeable truth" [171, 3, Objection 1], while the latter is not, and can cover a falsehood. Now the former foreknowledge, and not the latter, properly belongs to prophecy, because, as stated above (Question 171, Article 3), prophetic knowledge is of things which naturally surpass human knowledge. Consequently we must say that prophecy strictly so called cannot be from nature, but only from Divine revelation.
Reply to Objection 1. When the soul is withdrawn from corporeal things, it becomes more adapted to receive the influence of spiritual substances [Cf. I, 88, 4, ad 2], and also is more inclined to receive the subtle motions which take place in the human imagination through the impression of natural causes, whereas it is hindered from receiving them while occupied with sensible things. Hence Gregory says (Dial. iv, 26) that "the soul, at the approach of death, foresees certain future things, by reason of the subtlety of its nature," inasmuch as it is receptive even of slight impressions. Or again, it knows future things by a revelation of the angels; but not by its own power, because according to Augustine (Gen. ad lit. xii, 13), "if this were so, it would be able to foreknow the future whenever it willed," which is clearly false.
Objection 2. Knowledge of the future by means of dreams, comes either from the revelation of spiritual substances, or from a corporeal cause, as stated above (Question 95, Article 6), when we were treating of divination. Now both these causes are more applicable to a person while asleep than while awake, because, while awake, the soul is occupied with external sensibles, so that it is less receptive of the subtle impressions either of spiritual substances, or even of natural causes; although as regards the perfection of judgment, the reason is more alert in waking than in sleeping.
Reply to Objection 3. Even dumb animals have no foreknowledge of future events, except as these are foreknown in their causes, whereby their imagination is moved more than man's, because man's imagination, especially in waking, is more disposed according to reason than according to the impression of natural causes. Yet reason effects much more amply in man, that which the impression of natural causes effects in dumb animals; and Divine grace by inspiring the prophecy assists man still more.
Reply to Objection 4. The prophetic light extends even to the direction of human acts; and in this way prophecy is requisite for the government of a people, especially in relation to Divine worship; since for this nature is not sufficient, and grace is necessary.
Source: Summa Theologica (New Advent)