Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

5 The purpose in the heart of a man is deep water; But a man of understanding draweth it out. "Still waters are deep." Like such deep waters (Pro 18:4) is that which a man hath secretly (Isa 29:15) planned in his heart. He keeps it secret, conceals it carefully, craftily misleads those who seek to draw it out; but the man of תּבוּנה, i.e., one who possesses the right criteria for distinguishing between good and bad, true and false, and at the same time has the capacity to look through men and things, draws out (the Venet. well, ἀνέλξει) the secret עצה, for he penetrates to the bottom of the deep water. Such an one does not deceive himself with men, he knows how to estimate their conduct according to its last underlying motive and aim; and if the purpose is one that is pernicious to him, he meets it in the process of realization. What is here said is applicable not only to the subtle statesman and the general, but also the pragmatical historian and the expositor, as, e.g., of a poem such as the book of Job, the idea of which lies like a pearl at the bottom of deep water.