The verse before us addresses the wise person's power of speech: how the heart guides the mouth, and how the act of speech itself adds understanding to a person. We will first examine the explanations of the Rishonim on the verse, and then the approach of the Malbim, who sees the verse as a direct continuation of what we previously learned regarding the essence of wisdom.
לֵב חָכָם יַשְׂכִּיל פִּיהוּ וְעַל שְׂפָתָיו יֹסִיף לֶקַח׃ (כג) - The heart of the wise instructs his mouth, and adds learning to his lips. (23)
Rashi explains: the heart of the wise person teaches his mouth to speak with eloquence and clarity. By what virtue does the wise person speak in clear, articulate language? By virtue of his heart. Clarity in speech is not merely an external talent, but the result of internal clarity in the heart.
Rabbeinu Yonah explains that the wise person thinks before he speaks, and when he speaks, his heart adds insight to the expression of his lips, beyond the good sense and understanding he formulated even before speaking. Thus, the wise person possesses two distinct virtues. The first virtue: he does not simply blurt out whatever he thinks. Before uttering a word, he checks - is this fit to be said or not, might anyone be hurt, and what is the proper way to state it? The second virtue: during the speech itself, ideas arise in his mind, the concepts develop within him, and sometimes, precisely as he expresses the words orally, he receives a fresh insight, the matters become clearer to him, and he conveys additional understanding.
This is what the verse means: "The heart of the wise instructs his mouth" - one who is wise, his heart imparts prudence to the mouth before he speaks: what to say, how to say it, and in what manner to convey the message. "And adds learning to his lips" - even at the moment when the words are already on his lips, further learning is added to him.
Rav Shalom Schwadron would say that he had a tradition from his teachers: when a person is delivering a derashah and suddenly recalls a story in the middle, he should tell it. Heaven brought this story to his mind at that exact moment because it may inspire a specific listener, and it is not by chance. This is "and adds learning to his lips" - in the midst of speaking, when the words are already upon the lips, new insights, new stories, and thoughts he had not previously considered are granted to him.
The Vilna Gaon explains: "The heart of the wise instructs his mouth" - this refers to a person who, before uttering anything, strives to understand it thoroughly. Herein lies the difference between a person who attends a shiur and understands, versus one who attends and does not understand. Sometimes it depends on us, that our comprehension falls short; but often it depends on how well the maggid shiur prepared the lesson. If the maggid shiur has a crystal-clear understanding of the material - assuming he has reasonable explanatory skills - you will understand what he conveys. But if he himself is not completely clear about what he is delivering, how can he clarify the matter so that it becomes lucid and illuminating for you?
The Gra further adds: the wise person constantly reviews and rehearses what he needs to say. First, he ponders, reflects, reviews, and rehearses the material, and only then - "and adds learning to his lips" - when he articulates the words on his lips, he adds learning. By virtue of this review, the matters will become clearer and far better understood by the listeners as well.
The Malbim explains that this verse continues what we learned previously. Who is considered a wise person? One whose wisdom has become an acquired, integral part of him, until he is called "wise of heart." Wisdom, by its very nature, is received - handed down from previous generations, received as Torah from Sinai. Wisdom defines the upright path and morality: what is forbidden and what is permitted, what is moral and what is immoral - and this comes solely from what we have received. After all, what seems moral to one person may seem immoral to another. These matters do not depend on human reason; rather, they are an axiom received from the Creator of the universe, Who formed man and Whose wisdom is boundless and without end.
It follows that it makes no sense at all for a person without Torah and mitzvos to claim: "I am a moral person." Who decided what is moral? Even Hitler thought he was moral. The determination of what is moral and what is immoral must come from a Source above you.
Once a person attains the status of "wise of heart" - and the heart is what directs a person, guiding him continuously in the ways of wisdom until that wisdom becomes an acquired part of his essence - such a person will find that "his mouth will gain insight." We have already learned that the Malbim distinguishes between the mouth and the lips: the mouth always represents the more internal dimension, corresponding to chochmah (wisdom); whereas the lips represent the external dimension, corresponding to da'as (knowledge). Wisdom is hidden and internal, something one must receive from another, for one cannot arrive at the foundations of morality through independent intellect alone. Da'as, on the other hand, is complete and clear knowledge.
Says the Malbim: when a person speaks of the laws of wisdom, delves into them, reflects on them, and contemplates them - then "upon his lips he adds learning." When you enter into them and probe deeply, the concepts begin to make sense: you suddenly perceive the logic and rationale behind matters that you originally accepted from another as an axiom rather than something intuitively understood. You delved into them, meditated on them, and this transformed into the level of "his mouth will gain insight," until it reached "upon his lips he adds learning," which is clear and absolute knowledge.
Not only that, but the Malbim adds that he will even generate new insights and teachings within the laws of wisdom. Wisdom, as mentioned, is received tradition; but if you delve into it and contemplate it - contemplation being the aspect of binah (understanding), deriving and comprehending one thing from another - you arrive at da'as, a clear and definitive knowledge of the matter. Then you suddenly perceive further insights which you did not directly receive through tradition, but rather derived through logical deduction as a result of your deep contemplation. All of this stems from the person having internalized and applied the wisdom within him, until it became wisdom of the heart that governs the entire body, and from there he arrived at understanding, and from understanding to da'as.
"The heart of the wise instructs his mouth, and adds learning to his lips" - the wise person does not simply blurt out whatever comes to mind; rather, his heart instructs his mouth: what to say, how to say it, and to whom. He understands the matter thoroughly and reviews it before articulating it, so his words emerge polished and clear to his listeners as well. And as he speaks, additional insights and illustrations come to him that he had not previously contemplated - a gift from Heaven. According to the Malbim: wisdom comes to us as a received tradition, but one who delves into it and contemplates it with understanding elevates it from the level of "mouth" to the level of "lips" - from received tradition to clear, definitive knowledge - and even innovates within it new teachings that he never received from anyone.