I won’t believe it without proof. This expression, signifying shrewd native skepticism rather than provincial stupidity, has been traced to a number of sources. The oldest source of the thought suggested to date is the Missouri Compromise of 1820, a proviso that the constitution of the then new state would not prohibit slavery, which was reached after several years of dickering. As for the wording, one writer cites a speech made by Congressman Willard D. Vandiver in 1899; another refers to a song from the same period, “I’m from Missouri and You’ve Got to Show Me,” with lyrics by Lee Raney and music by Ned Wayburn. Thomas Oliphant, describing Senator Edward Kennedy’s doubts about a Supreme Court nominee, wrote, “Kennedy has become the leading Show Me Senator” (Boston Globe, Sept. 29, 2005).