Iron sharpeneth iron; so a man sharpeneth the countenance of his friend.
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Proverbs 27:15-19
Tyndale Open Study Notes (CC BY-SA 4.0).
27:17 iron sharpens iron: Wisdom comes from interaction, often critical, with a good friend (see 27:6, 9).
Other Bible passages linked to this one, most cited first.
And let us consider one another to provoke unto love and to good works:
Ointment and perfume rejoice the heart: so doth the sweetness of a man’s friend by hearty counsel.
Behold, thou hast instructed many, and thou hast strengthened the weak hands. Thy words have upholden him that was falling, and thou hast strengthened the feeble knees.
And Jonathan Saul’s son arose, and went to David into the wood, and strengthened his hand in God.
Strengthen ye the weak hands, and confirm the feeble knees. Say to them that are of a fearful heart, Be strong, fear not: behold, your God will come with vengeance, even God with a recompence; he will come and save you.
Thou therefore endure hardness, as a good soldier of Jesus Christ.
Authorship: the book itself or Christian tradition. Approximate dates, as in Carson and Moo, An Introduction to the New Testament, and Longman and Dillard, An Introduction to the Old Testament.