Objection 1. It would seem that folly is not contrary to wisdom. For seemingly unwisdom is directly opposed to wisdom. But folly does not seem to be the same as unwisdom, for the latter is apparently about Divine things alone, whereas folly is about both Divine and human things. Therefore folly is not contrary to wisdom.
Objection 2. Further, one contrary is not the way to arrive at the other. But folly is the way to arrive at wisdom, for it is written (1 Corinthians 3:18): "If any man among you seem to be wise in this world, let him become a fool, that he may be wise." Therefore folly is not opposed to wisdom.
Objection 3. Further, one contrary is not the cause of the other. But wisdom is the cause of folly; for it is written (Jeremiah 10:14): "Every man is become a fool for knowledge," and wisdom is a kind of knowledge. Moreover, it is written (Isaiah 47:10): "Thy wisdom and thy knowledge, this hath deceived thee." Now it belongs to folly to be deceived. Therefore folly is not contrary to wisdom.
Objection 4. Further, Isidore says (Etym. x, under the letter S) that "a fool is one whom shame does not incite to sorrow, and who is unconcerned when he is injured." But this pertains to spiritual wisdom, according to Gregory (Moral. x, 49). Therefore folly is not opposed to wisdom.
On the contrary, Gregory says (Moral. ii, 26) that "the gift of wisdom is given as a remedy against folly."
I answer that, Stultitia [Folly] seems to take its name from "stupor"; wherefore Isidore says (Etym. x, under the letter of S): "A fool is one who through dullness [stuporem] remains unmoved." And folly differs from fatuity, according to the same authority (Etym. x), in that folly implies apathy in the heart and dullness in the senses, while fatuity denotes entire privation of the spiritual sense. Therefore folly is fittingly opposed to wisdom.
For "sapiens" [wise] as Isidore says (Etym. x) "is so named from sapor [savor], because just as the taste is quick to distinguish between savors of meats, so is a wise man in discerning things and causes." Wherefore it is manifest that "folly" is opposed to "wisdom" as its contrary, while "fatuity" is opposed to it as a pure negation: since the fatuous man lacks the sense of judgment, while the fool has the sense, though dulled, whereas the wise man has the sense acute and penetrating.
Reply to Objection 1. According to Isidore (Etym. x), "unwisdom is contrary to wisdom because it lacks the savor of discretion and sense"; so that unwisdom is seemingly the same as folly. Yet a man would appear to be a fool chiefly through some deficiency in the verdict of that judgment, which is according to the highest cause, for if a man fails in judgment about some trivial matter, he is not for that reason called a fool.
Reply to Objection 2. Just as there is an evil wisdom, as stated above (45, 1, ad 1), called "worldly wisdom," because it takes for the highest cause and last end some worldly good, so too there is a good folly opposed to this evil wisdom, whereby man despises worldly things: and it is of this folly that the Apostle speaks.
Reply to Objection 3. It is the wisdom of the world that deceives and makes us foolish in God's sight, as is evident from the Apostle's words (1 Corinthians 3:19).
Reply to Objection 4. To be unconcerned when one is injured is sometimes due to the fact that one has no taste for worldly things, but only for heavenly things. Hence this belongs not to worldly but to Divine wisdom, as Gregory declares (Moral. x, 49). Sometimes however it is the result of a man's being simply stupid about everything, as may be seen in idiots, who do not discern what is injurious to them, and this belongs to folly simply.
Source: Summa Theologica (New Advent)