Therefore I prayed, and understanding was given me; I called upon God, and the spirit of wisdom came to me. I preferred her to scepters and thrones, and I accounted wealth as nothing in comparison with her. Neither did I liken to her any priceless gem, because all gold is but a little sand in her sight, and silver will be accounted as clay before her. I loved her more than health and beauty, and I chose to have her rather than light, because her radiance never ceases. All good things came to me along with her, and in her hands uncounted wealth. I rejoiced in them all, because wisdom leads them; but I did not know that she was their mother. I learned without guile and I impart without grudging; I do not hide her wealth, for it is an unfailing treasure for men; those who get it obtain friendship with God, commended for the gifts that come from instruction. — Wisdom 7:7-14
That the Gift of Understanding Corresponds to the Sixth Beatitude (i.e., "Blessed are the pure of heart...")
Augustine says (De Serm. Dom. in Monte i, 4): "The sixth work of the Holy Ghost which is understanding, is applicable to the clean of heart, whose eye being purified, they can see what eye hath not seen."
Two things are contained in the sixth beatitude, as also in the others, one by way of merit, viz. cleanness of heart; the other by way of reward, viz. the sight of God, as stated above (I-II, 69, 2,4), and each of these, in some way, responds to the gift of understanding.
For cleanness is twofold. One is a preamble and a disposition to seeing God, and consists in the heart being cleansed of inordinate affections: and this cleanness of heart is effected by the virtues and gifts belonging to the appetitive power. The other cleanness of heart is a kind of complement to the sight of God; such is the cleanness of the mind that is purged of phantasms and errors, so as to receive the truths which are proposed to it about God, no longer by way of corporeal phantasms, nor infected with heretical misrepresentations: and this cleanness is the result of the gift of understanding.
Again, the sight of God is twofold. One is perfect, whereby God's Essence is seen: the other is imperfect, whereby, though we see not what God is, yet we see what He is not; and whereby, the more perfectly do we know God in this life, the more we understand that He surpasses all that the mind comprehends. Each of these visions of God belongs to the gift of understanding; the first, to the gift of understanding in its state of perfection, as possessed in heaven; the second, to the gift of understanding in its state of inchoation, as possessed by wayfarers.