Must the Speaker of the House be a Member of Congress?
At Balkinization, Sanford Levinson says no. Akhil Amar says yes (maybe). And from the comments:
I don't see any constitutional basis for Professor Amar's suggestion that the Speaker has to be a member of the House. No doubt the framers expected that this would normally, if not invariably, be the case, but there is no obvious reason why they would have made it a requirement, and I can't see any reason to read it into the Constitution's silence on the point.
(The relevant text is, from Article II, Section 2, "The House of Representatives shall chuse their Speaker and other Officers").
A lot has been made in academic writing of the difference between original meaning and original expectations. At minimum, it seems right that there are many things the framers expected to happen, but which they did not write into the Constitution's text. A textualist originalist would say, I think, that (generally) those things are not constitutionally required, while an intentionalist might say that they are.