The contrast between the wisdom of him who trusts in the Lord, and the folly of self-trust.

He that trusteth in his own heart is a fool - For his heart, which is deceitful and desperately wicked, will infallibly deceive him.

He that trusteth in his own heart is a fool,.... Since the thoughts and imaginations of the thoughts of the heart are only evil, and that continually; they are vain and vague, sinful and corrupt; the affections are inordinate, the conscience defiled, the understanding darkened, and the will perverse; there is no good thing in it, nor any that comes out of it, but all the reverse; it is deceitful and desperately wicked: he must be a fool, and not know the plague of his heart, that trusts in it; and even for a good man to be self-confident, and trust to the sincerity of his heart, as Peter did, or to the good frame of the heart, as many do, is acting a foolish part; and especially such are fools as the Scribes and Pharisees, who trusted in themselves that they were righteous, and despised others, when a man's best righteousness is impure and imperfect, and cannot justify him in the sight of God; it is moreover a weak and foolish part in men to trust to the wisdom and counsel of their heart, to lean to their own understanding, even it, things natural and civil, and not to ask wisdom of God, or take the advice of men, and especially it, things religious and sacred; see Proverbs 3:5;
but whoso walketh wisely; as he does who walks according to the rule of the divine word; who makes the testimonies of the Lord his counsellors; who consults with his sacred writings, and follows the directions of them; who walks as he has Christ for his pattern and example, and makes the Spirit of God his guide, and walks after him, and not after the flesh; who walks with wise men, and takes their advice in all matters of moment, not trusting to his own wisdom and knowledge; who walks as becomes the Gospel of Christ, and in all the ordinances of it; who walks inoffensively to all men, and so in wisdom towards them that are without, and in love to them who are within; who walks circumspectly, not as fools, but as wise, redeeming the time;
he shall be delivered; he shall be delivered from the snares of his own deceitful heart, which he will not trust; and from the temptations of Satan; and from all afflictions and troubles he meets with in the way; and from a final and total falling away; and from eternal death and destruction: "he shall be saved", as some versions render it, even with an everlasting salvation. The Targum is,
"he shall be protected from evil.''

A fool trusts to his own strength, merit, and righteousness. And trusts to his own heart, which is not only deceitful above all things, but which has often deceived him.

(Compare Proverbs 3:6-8).
walketh wisely--that is, trusting in God (Proverbs 22:17-19).

The following proverb assumes the בטח of the foregoing:
(Note: We take the opportunity of remarking that the tendency to form together certain proverbs after one catchword is found also in German books of proverbs; vid., Paul, Ueber die urspr. Anord. von Friedanks Bescheidenheit (1870), p. 12.)
26 He that trusteth in his own heart is a fool;
But he that walketh in wisdom shall escape.
From the promise in the second line, Hitzig concludes that a courageous heart is meant, but when by itself לב never bears this meaning. He who trusteth in his own heart is not merely one who is guided solely "by his own inconsiderate, defiant impulse to act" (Zckler). The proverb is directed against a false subjectivity. The heart is that fabricator of thoughts, of which, as of man by nature, nothing good can be said, Genesis 6:5; Genesis 8:21. But wisdom is a gift from above, and consists in the knowledge of that which is objectively true, that which is normatively godlike. הלך בּחכמה is he who so walks that he has in wisdom a secure authority, and has not then for the first time, when he requires to walk, need to consider, to reckon, to experiment. Thus walking in the way of wisdom, he escapes dangers to which one is exposed who walks in foolish confidence in his own heart and its changeful feelings, thoughts, imaginations, delusions. One who thoughtlessly boasts, who vainly dreams of victory before the time, is such a person; but confidence in one's own heart takes also a hundred other forms. Essentially similar to this proverb are the words of Jeremiah 9:22., for the wisdom meant in 26b is there defined at Jeremiah 9:23.

Wisely - Distrusting his own judgment, and seeking the advice of others, and especially of God.