The Constitution also defines two, and only two forms of citizenship - natural born and naturalized. If we agree that there are only two forms of citizenship and that one of them is naturalized then it kind of narrows down the other form, doesn't it?
The term has always been interpreted to mean a child born on US Soil of two US citizens...
Not by any law or clause of the Constitution or legal decision I'm aware of.
That's right -- HE