Give instruction to a wise man, and he will be yet wiser. The Hebrew is merely "give to the wise," with no object mentioned; but the context suggests "instruction," even though, as in Proverbs 9:8, it takes the form of rebuke.
Vulgate and Septuagint, "Give an opportunity to a wise man, and he will be wiser" (comp. Matthew 13:12; Matthew 25:29). To make the best use of all occasions of learning duty, whether they present themselves in a winning or a forbidden shape, is the part of one who is wise unto salvation (see Proverbs 1:5, and note there).
Teach a just man, and he will increase in learning. Wisdom being a moral and not merely an intellectual, quality. there is a natural interchange of "wise" and "just," referring to the same individual, in the two clauses.
Vulgate, festinabit accipere; Septuagint, "Instruct a wise man, and he shall have more given him." The wise are thus rewarded with larger measures of wisdom, because they are simple, humble, and willing to learn, having that childlike spirit which Christ commends (Matthew 18:3).