" The Constitution is silent on that question, saying simply, “The House of Representatives shall chuse (sic) their Speaker and other Officers.”
The Clerk of the House agrees with the office of the House Historian, which says the speaker “has always been (but is not required to be) a House Member.”
Most historians and legal experts who’ve looked at this issue conclude the founders simply assumed the speaker would be drawn from among elected members.
“It would have been unthinkable for the most populous house not to have its leader be part of the representatives who were elected by the people,” says David Forte, a constitutional scholar at Cleveland State University.
“Nothing fits that would make the speaker anything other than a member of the house,” except for the Constitution’s silence on the issue, Forte says, noting that the Articles of Confederation said members of Congress shall have authority “to appoint one of their members to preside.” "
Gotta love how the vagueness of the Founding Fathers is cause for serious debate after they just assumed something was obvious.