Sprüche 12:15

15 The way of the fool is right in his own eyes, But the wise listeneth to counsel. Other proverbs, like Pro 16:2, say that generally the judgment of a man regarding his character does not go beyond a narrow subjectivity; but there are objective criteria according to which a man can prove whether the way in which he walks is right; but the fool knows not other standard than his own opinion, and however clearly and truly one may warn him that the way which he has chosen is the wrong way and leads to a false end, yet he obstinately persists; (Note: Vid., kindred proverbs by Carl Schulze, Die bibl. Sprichwrter der deutschen Sprache (1860), p. 50, and M. C. Wahl's Das Sprichwort in der heb.-aram. Literatur, u.s.w. (1871), p. 31.) while a wise man is not so wise in his own eyes (Pro 3:7) as not to be willing to listen to well-meant counsel, because, however careful he may be regarding his conduct, yet he does not regard his own judgment so unerring as not to be inclined ever anew to try it and let it stand the test. Ewald has falsely construed: yet whoever hears counsel is wise. In consequence of the contrast, אויל and חכם are the subject ideas, and with ושׁמע לעצה is brought forward that which is in contrast to the self-complacency of the fool, the conduct of the wise man.

See here, 1. What it is that keeps a fool from being wise: His way is right in his own eyes; he thinks he is in the right in every thing he does, and therefore asks no advice, because he does not apprehend he needs it; he is confident he knows the way, and cannot miss it, and therefore never enquires the way. The rule he goes by is to do that which is right in his own eyes, to walk in the way of his heart. Quicquid libet, licet - He makes his will his law. He is a fool that is governed by his eye, and not by his conscience. 2. What it is that keeps a wise man from being a fool; he is willing to be advised, desires to have counsel given him, and hearkens to counsel, being diffident of his own judgment and having a value for the direction of those that are wise and good. He is wise (it is a sign he is so, and he is likely to continue so) whose ear is always open to good advice.

12:15 Fools fail to realize what the wise know—that listening is the way to learn.