Objection 1. It would seem that prophecy is a habit. For according to Ethic. ii, 5, "there are three things in the soul, power, passion, and habit." Now prophecy is not a power, for then it would be in all men, since the powers of the soul are common to them. Again it is not a passion, since the passions belong to the appetitive faculty, as stated above (I-II, 22, 2); whereas prophecy pertains principally to knowledge, as stated in the foregoing Article. Therefore prophecy is a habit.
Objection 2. Further, every perfection of the soul, which is not always in act, is a habit. Now prophecy is a perfection of the soul; and it is not always in act, else a prophet could not be described as asleep. Therefore seemingly prophecy is a habit.
Objection 3. Further, prophecy is reckoned among the gratuitous graces. Now grace is something in the soul, after the manner of a habit, as stated above (I-II, 110, 2). Therefore prophecy is a habit.
On the contrary, A habit is something "whereby we act when we will," as the Commentator [Averroes or Ibn Roshd, 1120-1198] says (De Anima iii). But a man cannot make use of prophecy when he will, as appears in the case of Eliseus (2 Kings 3:15), "who on Josaphat inquiring of him concerning the future, and the spirit of prophecy failing him, caused a minstrel to be brought to him, that the spirit of prophecy might come down upon him through the praise of psalmody, and fill his mind with things to come," as Gregory observes (Hom. i super Ezech.). Therefore prophecy is not a habit.
I answer that, As the Apostle says (Ephesians 5:13), "all that is made manifest is light," because, to wit, just as the manifestation of the material sight takes place through material light, so too the manifestation of intellectual sight takes place through intellectual light. Accordingly manifestation must be proportionate to the light by means of which it takes place, even as an effect is proportionate to its cause. Since then prophecy pertains to a knowledge that surpasses natural reason, as stated above (Article 1), it follows that prophecy requires an intellectual light surpassing the light of natural reason. Hence the saying of Micah 7:8: "When I sit in darkness, the Lord is my light." Now light may be in a subject in two ways: first, by way of an abiding form, as material light is in the sun, and in fire; secondly, by way of a passion, or passing impression, as light is in the air. Now the prophetic light is not in the prophet's intellect by way of an abiding form, else a prophet would always be able to prophesy, which is clearly false. For Gregory says (Hom. i super Ezech.): "Sometimes the spirit of prophecy is lacking to the prophet, nor is it always within the call of his mind, yet so that in its absence he knows that its presence is due to a gift." Hence Eliseus said of the Sunamite woman (2 Kings 4:27): "Her soul is in anguish, and the Lord hath hid it from me, and hath not told me." The reason for this is that the intellectual light that is in a subject by way of an abiding and complete form, perfects the intellect chiefly to the effect of knowing the principle of the things manifested by that light; thus by the light of the active intellect the intellect knows chiefly the first principles of all things known naturally. Now the principle of things pertaining to supernatural knowledge, which are manifested by prophecy, is God Himself, Whom the prophets do not see in His essence, although He is seen by the blessed in heaven, in whom this light is by way of an abiding and complete form, according to Psalm 35:10, "In Thy light we shall see light."
It follows therefore that the prophetic light is in the prophet's soul by way of a passion or transitory impression. This is indicated Exodus 33:22: "When my glory shall pass, I will set thee in a hole of the rock," etc., and 1 Kings 19:11: "Go forth and stand upon the mount before the Lord; and behold the Lord passeth," etc. Hence it is that even as the air is ever in need of a fresh enlightening, so too the prophet's mind is always in need of a fresh revelation; thus a disciple who has not yet acquired the principles of an art needs to have every detail explained to him. Wherefore it is written (Isaiah 1:4): "In the morning He wakeneth my ear, so that I may hear Him as a master." This is also indicated by the very manner in which prophecies are uttered: thus it is stated that "the Lord spake to such and such a prophet," or that "the word of the Lord," or "the hand of the Lord was made upon him."
But a habit is an abiding form. Wherefore it is evident that, properly speaking, prophecy is not a habit.
Reply to Objection 1. This division of the Philosopher's does not comprise absolutely all that is in the soul, but only such as can be principles of moral actions, which are done sometimes from passion, sometimes from habit, sometimes from mere power, as in the case of those who perform an action from the judgment of their reason before having the habit of that action.
However, prophecy may be reduced to a passion, provided we understand passion to denote any kind of receiving, in which sense the Philosopher says (De Anima iii, 4) that "to understand is, in a way, to be passive." For just as, in natural knowledge, the possible intellect is passive to the light of the active intellect, so too in prophetic knowledge the human intellect is passive to the enlightening of the Divine light.
Reply to Objection 2. Just as in corporeal things, when a passion ceases, there remains a certain aptitude to a repetition of the passion--thus wood once ignited is more easily ignited again, so too in the prophet's intellect, after the actual enlightenment has ceased, there remains an aptitude to be enlightened anew--thus when the mind has once been aroused to devotion, it is more easily recalled to its former devotion. Hence Augustine says (De orando Deum. Ep. cxxx, 9) that our prayers need to be frequent, "lest devotion be extinguished as soon as it is kindled."
We might, however, reply that a person is called a prophet, even while his prophetic enlightenment ceases to be actual, on account of his being deputed by God, according to Jeremiah 1:5, "And I made thee a prophet unto the nations."
Reply to Objection 3. Every gift of grace raises man to something above human nature, and this may happen in two ways. First, as to the substance of the act--for instance, the working of miracles, and the knowledge of the uncertain and hidden things of Divine wisdom--and for such acts man is not granted a habitual gift of grace. Secondly, a thing is above human nature as to the mode but not the substance of the act--for instance to love God and to know Him in the mirror of His creatures--and for this a habitual gift of grace is bestowed.
Source: Summa Theologica (New Advent)