This is a tighter requirement than simply citizen or naturalized citizen, just like citizen at least 35 years old is a tighter requiremeet than just citizen. So, natural born is an understood requirement for office, not a Constitutional definition of who is a citizen.
What if Article II were instead written as:
"No person except a Citizen, natural born, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States."
We don't argue that "citizen over 35" or "14-year resident citizen" are additional forms of citizenship, so why the insistence that "natural born citizen" IS a form of citizenship to be rejected, when the context is clearly to further qualify what type of citizen is eligible to be President?
-PJ
Because it's not hard to define "citizen over 35" or "14-year resident citizen". But you cannot point to a definition of "natural-born citizen". And considering that the Constitution was written at a time when some nations defined it jus solis and some defined it jus sanguinis then your claim that using the "common understanding" should suffice doesn't hold water. Common understanding held by who?