Geoffrey Chaucer Quote

A whetstone is no carving instrument, And yet it maketh sharp the carving tool; And if you see my efforts wrongly spent, Eschew that course and learn out of my school; For thus the wise may profit by the fool, And edge his wit, and grow more keen and wary, For wisdom shines opposed to its contrary.

Troilus and Cressida and The Canterbury tales: With modern English versions of both works (ed. 1952)