Useth knowledge aright - Rather, makes knowledge goodly. The power of well-considered speech to commend true wisdom, is contrasted with the pouring (literally as in the margin) forth of folly.

Useth knowledge aright - This is very difficult to know: - when to speak, and when to be silent; what to speak, and what to leave unspoken; the manner that is best and most suitable to the occasion, the subject, the circumstances, and the persons. All these are difficulties, often even to the wisest men. Even wise counsel may be foolishly given.

The tongue of the wise useth knowledge aright,.... As the heart of a wise and good man is filled with useful knowledge, civil, moral, spiritual, and evangelical; so he takes care to communicate it, at proper times and seasons, in proper places, and to proper persons; adapting it to their case and circumstances, so as it may be for their comfort, edification, and instruction, and minister grace unto them; which is using knowledge "well", as the word (s) signifies: such an use of it recommends it, and makes it appear beautiful and lovely, decorates and adorns it. Thus every good man, out of the good treasure of knowledge in his heart, brings forth his good things seasonably, to the use of edifying; in like manner, ministers of the word, scribes well instructed in the things of God, bring forth both new and old, to the profit of those to whom they minister; so Christ, as man and Mediator, had the tongue of the learned, to speak a word in season to weary souls;
but the mouth of fools poureth out foolishness; their knowledge, as they take it to be, but it is no other than folly; this they throw out in great plenty, in a hurry, without fear or wit; they "babble" it out, as the word (t) signifies, as water out of a fountain; their hearts are full of it, and their mouths proclaim it, Proverbs 12:23.
(s) "utitur bene", Castalio; "pulchre", Vatablus. (t) "effutit, ebullit, fundit", Vatablus; "eructat", Junius & Tremellius, Piscator; "ebullit", V. L. Tigurine version, Schultens; "scaturire facit", Michaelis.

He that has knowledge, is to use it aright, for the good of others.

useth . . . aright--commends knowledge by its proper use.
poureth out--utters abundantly (Proverbs 12:23), and so disgusts others.

The πραΰ́της σοφίας (James 3:13) commended in Proverbs 15:1 is here continued:
The tongue of the wise showeth great knowledge,
And the mouth of fools poureth forth folly.
As היטיב נגּן, Isaiah 23:16, means to strike the harp well, and היטיב לכת, Isaiah 30:29, to go along merrily, so היטיב דּעת, to know in a masterly manner, and here, where the subject is the tongue, which has only an instrumental reference to knowledge: to bring to light great knowledge (cf. 7a). In 2b the lxx translate στόμα δὲ ἀφρόνων ἀναγγέλλει κακά. From this Hitzig concludes that they read רעות as 28b, and prefers this phrase; but they also translated in Proverbs 13:16; Proverbs 14:28; Proverbs 26:11, אוּלת by κακίαν, for they interpreted the unintelligible word by combination with עולת, and in Proverbs 12:23 by ἀραῖς, for they thought they had before them אלות (from אלה).

Aright - Expressing what he knows prudently and gracefully. Poureth out - Plentifully, continually, and vehemently, as a fountain does waters.