The heart of the wise teacheth his mouth - He has a wise heart; he speaks as it dictates; and therefore his speeches are all speeches of wisdom.

The heart of the wise teacheth his mouth,.... That is, a man that is wise in heart, as in Proverbs 16:21; his heart will teach his mouth what to say, when to say it, and before whom; it will prompt him to matter, that he shall not be at a loss what to say, nor how to say; it will furnish him with words and things; out of the abundance of the heart the mouth speaketh, Matthew 12:34;
and addeth learning to his lips: so that he does not deliver out mere words, but solid learning along with them, instructive to himself and others. The Targum is,
"and by his lips he addeth doctrine;''
or increases knowledge.

The wise man's self-knowledge, always suggests something proper to be spoken to others.

The heart is the source of wisdom flowing from the mouth.

23 The heat of the wise maketh his mouth wise,
And learning mounteth up to his lips.
Regarding השׂכּיל as causative: to put into the possession of intelligence, vid., at Genesis 3:6. Wisdom in the heart produceth intelligent discourse, and, as the parallel member expresses it, learning mounteth up to the lips, i.e., the learning which the man taketh into his lips (Proverbs 22:18; cf. Psalm 16:4) to communicate it to others, for the contents of the learning, and the ability to communicate it, are measured by the wisdom of the heart of him who possesses it. One can also interpret הוסיף as extens. increasing: the heart of the wise increaseth, i.e., spreads abroad learning, but then בּשׂפתיו (Psalm 119:13) would have been more suitable; על־שׂפתיו calls up the idea of learning as hovering on the lips, and thus brings so much nearer, for הוסיף, the meaning of the exaltation of its worth and impression.

Teacheth - Directs him what to speak, and keeps him from speaking foolishly. Addeth - Enables him to speak learnedly and wisely.