The Great Preface to the Book of Songs explains one of its kinds of poem, feng (风). “Those above use it to transform those below. Those below use it to criticize those above. It leads with art and remonstrates indirectly. The one who speaks is not guilty, and the one who hears is warned enough.” (上以风化下,下以风刺上,主文而谲谏,言之者无罪,闻之者足以戒)
The last line lives on as the saying 言者无罪,闻者足戒. Dictionaries define it this way: the person who raises an opinion is not at fault, and the person who hears it should take it as a warning.