{i} The heart is deceitful above all things, and desperately wicked: who can know it?

(i) Because the wicked always have some excuse to defend their doings he shows that their own lewd imaginations deceive them and bring them to these hardships: but God will examine their deeds by the malice of their hearts, 1Sa 16:7,1Ch 28:9,Ps 7:10,Jer 11:20,10:12,Re 2:13.

Wesley's Notes

17:9 The heart - There is nothing so false and deceitful as the heart of man; deceitful in its apprehensions of things, in the hopes and promises which it nourishes, in the assurances that it gives us; unsearchable by others, deceitful with reference to ourselves, and abominably wicked, so that neither can a man know his own heart, nor can any other know that of his neighbour's.

Jamieson-Fausset-Brown Bible Commentary

9. deceitful-from a root, "supplanting," "tripping up insidiously by the heel," from which Jacob (Ho 12:3) took his name. In speaking of the Jews' deceit of heart, he appropriately uses a term alluding to their forefather, whose deceit, but not whose faith, they followed. His "supplanting" was in order to obtain Jehovah's blessing. They plant Jehovah for "trust in man" (Jer 17:5), and then think to deceive God, as if it could escape His notice, that it is in man, not in Him, they trust.

desperately wicked-"incurable" [Horsley], (Mic 1:9). Trust in one's own heart is as foolish as in our fellow man (Pr 28:26).

Matthew Henry's Concise Commentary

17:5-11 He who puts confidence in man, shall be like the heath in a desert, a naked tree, a sorry shrub, the product of barren ground, useless and worthless. Those who trust to their own righteousness and strength, and think they can do without Christ, make flesh their arm, and their souls cannot prosper in graces or comforts. Those who make God their Hope, shall flourish like a tree always green, whose leaf does not wither. They shall be fixed in peace and satisfaction of mind; they shall not be anxious in a year of drought. Those who make God their Hope, have enough in him to make up the want of all creature-comforts. They shall not cease from yielding fruit in holiness and good works. The heart, the conscience of man, in his corrupt and fallen state, is deceitful above all things. It calls evil good, and good evil; and cries peace to those to whom it does not belong. Herein the heart is desperately wicked; it is deadly, it is desperate. The case is bad indeed, if the conscience, which should set right the errors of other faculties, is a leader in the delusion. We cannot know our own hearts, nor what they will do in an hour of temptation. Who can understand his errors? Much less can we know the hearts of others, or depend upon them. He that believes God's testimony in this matter, and learns to watch his own heart, will find this is a correct, though a sad picture, and learns many lessons to direct his conduct. But much in our own hearts and in the hearts of others, will remain unknown. Yet whatever wickedness there is in the heart, God sees it. Men may be imposed upon, but God cannot be deceived. He that gets riches, and not by right, though he may make them his hope, never shall have joy of them. This shows what vexation it is to a worldly man at death, that he must leave his riches behind; but though the wealth will not follow to another world, guilt will, and everlasting torment. The rich man takes pains to get an estate, and sits brooding upon it, but never has any satisfaction in it; by sinful courses it comes to nothing. Let us be wise in time; what we get, let us get it honestly; and what we have, use it charitably, that we may be wise for eternity.