Where?
Please be specific and cite the exact Article, Section and Clause which proves your assertion...The Constitution of the United States
They do that by Laws.
According to my understanding of the Constitution, in regards to citizenship, all Congress can do is "...establish an uniform Rule of Naturalization..." (Article 1, Section 8, Clause 4).
Now, that makes aliens into American citizens, but that Clause doesn't say Congress "defines" citizenship.
Any help greatly appreciated.
Oh, BTW...a natural born citizen needs no recourse to a positive law (laws passed by Congress are positive laws) like an alien does.
What we do know is that citizens are either citizens at birth, by the definition in law, or they have to follow naturalization procedures to become citizens.
If you do not have to be naturalized, you are natural born.