Politeness is the art of choosing among one’s real thoughts.

Germaine de Staël (1766-1817) Swiss-French writer, woman of letters, critic, salonist [Anne Louise Germaine de Staël-Holstein, Madame de Staël, Madame Necker]

Quoted in Abel Stevens, Madame de Staël, Vol. 1, ch. 4 “Early Character” (1880)
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Stated as a possible paraphrase: "It was a maxim with her that politeness is the art of choosing among one's real thoughts."