“Civility,” Keith Thomas notes, “was (and is) a slippery and unstable word.” “Civil” and “civilian” evoke the social life of a people not under military rule, the world of the civitas—the organized community—the only place, according to Aristotle and Cicero, where the good life is possible. While “courtesy” relates to the values of the court, “civility,” Thomas writes, is “the virtue of citizens”: in his Dictionary (1755), Samuel Johnson defined it as both “politeness” and “the state of…