Matthew Henry and continuators on Proverbs 20:5

5 Counsel in the heart of man is like deep water; but a man of understanding will draw it out.

A man's wisdom is here said to be of use to him for the pumping of other people, and diving into them, 1. To get the knowledge of them. Though men's counsels and designs are ever so carefully concealed by them, so that they are as deep water which one cannot fathom, yet there are those who by sly insinuations, and questions that seem foreign, will get out of them both what they have done and what they intend to do. Those therefore who would keep counsel must not only put on resolution, but stand upon their guard. 2. To get knowledge by them. Some are very able and fit to give counsel, having an excellent faculty of cleaving a hair, hitting the joint of a difficulty, and advising pertinently, but they are modest, and reserved, and not communicative; they have a great deal in them, but it is loth to come out. In such a case a man of understanding will draw it out, as wine out of a vessel. We lose the benefit we might have by the conversation of wise men for want of the art of being inquisitive.