Objection 1. It would seem that the third beatitude, "Blessed are they that mourn," does not correspond to the gift of knowledge. For, even as evil is the cause of sorrow and grief, so is good the cause of joy. Now knowledge brings good to light rather than evil, since the latter is known through evil: for "the straight line rules both itself and the crooked line" (De Anima i, 5). Therefore the aforesaid beatitude does not suitably correspond to the gift of knowledge.
Objection 2. Further, consideration of truth is an act of knowledge. Now there is no sorrow in the consideration of truth; rather is there joy, since it is written (Wisdom 8:16): "Her conversation hath no bitterness, nor her company any tediousness, but joy and gladness." Therefore the aforesaid beatitude does not suitably correspond with the gift of knowledge.
Objection 3. Further, the gift of knowledge consists in speculation, before operation. Now, in so far as it consists in speculation, sorrow does not correspond to it, since "the speculative intellect is not concerned about things to be sought or avoided" (De Anima iii, 9). Therefore the aforesaid beatitude is not suitably reckoned to correspond with the gift of knowledge.
On the contrary, Augustine says (De Serm. Dom. in Monte iv): "Knowledge befits the mourner, who has discovered that he has been mastered by the evil which he coveted as though it were good."
I answer that, Right judgment about creatures belongs properly to knowledge. Now it is through creatures that man's aversion from God is occasioned, according to Wisdom 14:11: "Creatures... are turned to an abomination... and a snare to the feet of the unwise," of those, namely, who do not judge aright about creatures, since they deem the perfect good to consist in them. Hence they sin by placing their last end in them, and lose the true good. It is by forming a right judgment of creatures that man becomes aware of the loss (of which they may be the occasion), which judgment he exercises through the gift of knowledge.
Hence the beatitude of sorrow is said to correspond to the gift of knowledge.
Reply to Objection 1. Created goods do not cause spiritual joy, except in so far as they are referred to the Divine good, which is the proper cause of spiritual joy. Hence spiritual peace and the resulting joy correspond directly to the gift of wisdom: but to the gift of knowledge there corresponds, in the first place, sorrow for past errors, and, in consequence, consolation, since, by his right judgment, man directs creatures to the Divine good. For this reason sorrow is set forth in this beatitude, as the merit, and the resulting consolation, as the reward; which is begun in this life, and is perfected in the life to come.
Reply to Objection 2. Man rejoices in the very consideration of truth; yet he may sometimes grieve for the thing, the truth of which he considers: it is thus that sorrow is ascribed to knowledge.
Reply to Objection 3. No beatitude corresponds to knowledge, in so far as it consists in speculation, because man's beatitude consists, not in considering creatures, but in contemplating God. But man's beatitude does consist somewhat in the right use of creatures, and in well-ordered love of them: and this I say with regard to the beatitude of a wayfarer. Hence beatitude relating to contemplation is not ascribed to knowledge, but to understanding and wisdom, which are about Divine things.
Source: Summa Theologica (New Advent)