Letter to Duc de Richelieu, 18 June 1744. Although this saying is now associated with Voltaire, he is obviously quoting an Italian proverb here. The French form, which he used later, is Le mieux est l'ennemi du bien.
Address at the Sorbonne, Paris, 23 Apr. 1910. Richard M. Nixon quoted this passage in his address to the nation announcing his decision to resign the presidency, 8 Aug. 1974. See Theodore Roosevelt 1; Theodore Roosevelt 2; Theodore Roosevelt 5