Truth never turns to rebuke falsehood; her own straightforwardness is the severest correction.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“If you light on the least vestige of truth, and it is the weight of the whole body still which stamps the faintest trace, an eternity will not suffice to extol it, while no evil is so huge, but you grudge to bestow on it a moment of hate. Truth never turns to rebuke falsehood; her own straightforwardness is the severest correction. Horace would not have written satire so well if he had not been inspired by it, as by a passion, and fondly cherished his vein. In his odes, the love always exceeds the hate, so that the severest satire still sings itself, and the poet is satisfied, though the folly be not corrected.” source