Bray - To pound wheat in a mortar with a pestle, in order to free the wheat from its husks and impurities, is to go through a far more elaborate process than threshing. But the folly of the fool is not thus to be got rid of. It sticks to him to the last; all discipline, teaching, experience seem to be wasted on him.

Though thou shouldest bray a fool - Leaving all other conjectures, of which commentators are full, I would propose, that this is a metaphor taken from pounding metallic ores in very large mortars, such as are still common in the East, in order that, when subjected to the action of the fire, the metal may be the more easily separated from the ore. However you may try, by precept or example, or both, to instruct a stupid man, your labor is lost; his foolishness cannot be separated from him. You may purge metals of all their dross; but you cannot purge the fool of his folly.

Though thou shouldest bray a fool in a mortar among wheat with a pestle,.... As the manna was, Numbers 11:8; and as wheat beat and bruised in a mortar, or ground in a mill, retains its own nature; so, let a wicked man be used ever so roughly or severely, by words, admonitions, reproofs, and counsels; or by deeds, by corrections and punishment, by hard words or blows, whether publicly or privately; in the midst of the congregation, as the Targum and Syriac version; or of the sanhedrim and council, as the Septuagint and Arabic versions;
yet will not his foolishness depart from him; his inbred depravity and natural malignity and folly will not remove, nor will he leave his course of sinning he has been accustomed to; he is stricken in vain, he will revolt more and more, Isaiah 1:5. Anaxarchus the philosopher was ordered by the tyrant Nicocreon to be pounded to death in a stone mortar with iron pestles (q), and which he endured with great patience.
(q) Laert. in Vit. Anaxarch. l. 9. p. 668.

Some are so bad, that even severe methods do not answer the end; what remains but that they should be rejected? The new-creating power of God's grace alone is able to make a change.

The obstinate wickedness of such is incurable by the heaviest inflictions.

22 Though thou bruise a fool in a mortar among grit with a pestle,
Yet would not his folly depart from him.
According to the best accredited accentuations, אם־תּכתּושׁ has Illuj. and בּמּכתּשׁ has Pazer, not Rebia, which would separate more than the Dechi, and disturb the sequence of the thoughts. The first line is long; the chief disjunctive in the sphere of the Athnach is Dechi of 'הר, this disjoins more than the Pazer of 'בּם, and this again more than the Legarmeh of את־האויל. The ה of הרפות does not belong to the stem of the word (Hitzig), but is the article; רפות (from רוּף, to shake, to break; according to Schultens, from רפת, to crumble, to cut in pieces, after the form קיטור, which is improbable) are bruised grains of corn (peeled grain, grit), here they receive this name in the act of being bruised; rightly Aquila and Theodotion, ἐν μέσῳ ἐμπτισσομένων (grains of corn in the act of being pounded or bruised), and the Venet. μέσον τῶν πτισανῶν.
(Note: The lxx translates ἐν μέσῳ συνεδρίου, and has thereby misled the Syr., and mediately the Targum.)
In בּעלי (thus to be written after Michlol 43b, not בּעלי, as Heidenheim writes it without any authority) also the article is contained. מכתשׁ is the vessel, and the ב of בעלי is Beth instrumenti; עלי (of lifting up for the purpose of bruising) is the club, pestle (Luther: stempffel = pounder); in the Mishna, Beza i. 5, this word denotes a pounder for the cutting out of flesh. The proverb interprets itself: folly has become to the fool as a second nature, and he is not to be delivered from it by the sternest discipline, the severest means that may be tried; it is not indeed his substance (Hitzig), but an inalienable accident of his substance.