Iron sharpeneth iron
A sword or knife made of iron is sharpened by it; so butchers sharpen their knives; so a man sharpeneth the countenance of his friend;
by conversation with him; thus learned men sharpen one another's minds, and excite each other to learned studies; Christians sharpen one another's graces, or stir up each other to the exercise of them, and the gifts which are bestowed on them, and to love and to good works. So Jarchi and Gersom understand it of the sharpening of men's minds to the learning of doctrine; but Aben Ezra, takes it in an ill sense, that as iron strikes iron and sharpens it, so a wrathful man irritates and provokes wrath in another. Some render the words, "as iron delighteth in iron, so a man rejoiceth the countenance of his friend", F9: by his company and conversation.