The framers were, alas, silent on that question.
Saying that implies that every word in the Constitution is void because there was no Amendment defining each word. Ridiculous logic. When they wrote the phrase they knew what it meant because it was a universal notion. Citizenship that is natural is unassailable because it touches all the pertinent bases. Both parents are citizens, and the birth occurred in the homeland. The social contract is automatic when these circumstances are established.