Manners must adorn knowledge, and smooth its way through the world. Like a great rough diamond, it may do very well in a closet by way of curiosity, and also for its intrinsic value.

Philip Dormer Stanhope, 4th Earl of Chesterfield (1694–1773), is best known for the posthumously published Letters written to his illegitimate son, Philip Stanhope, offering advice on education, conduct, and advancement in polite society. The remark about manners “adorning” knowledge reflects Chesterfield’s lifelong preoccupation with the social uses of learning: in an 18th‑century world of patronage, courts, and salons, erudition alone did not guarantee influence. His letters repeatedly urge that scholarship be paired with ease, tact, and conversational grace so that one’s abilities are accepted and welcomed rather than resisted. The “rough diamond” image fits his recurring contrast between intrinsic worth and socially effective polish.

Chesterfield argues that knowledge has real value, but without manners it often fails to achieve practical effect. “Manners” here means more than etiquette: it includes tact, self-command, and the ability to make others comfortable. The metaphor of a rough diamond suggests that raw intellectual power may be impressive in private (“in a closet”) yet remain socially inert unless cut and polished—i.e., expressed with civility and charm. The quote also implies a moral and strategic claim: learning should not be wielded as a weapon or display of superiority, because social friction can blunt its influence. In short, polish is presented as the vehicle that carries substance into the world.