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While the nation's legal scholars differ over the exact meaning of the Constitution's requirement that a person must be a "natural born citizen" to become president, they're unanimous in saying Ted Cruz is wrong about an important point.

"As a legal matter, the question is quite straightforward and settled law," Cruz has said. "People will continue to make political noise about it, but as a legal matter it is quite straightforward."

In fact, the experts say, it is neither settled nor straightforward.

It's not settled — because the Constitution does not define "natural born," a phrase that appears in the nation's founding document only once.

And though the federal courts have chewed on it from time to time, the U.S. Supreme Court has never officially said what it means.

It's not straightforward — because at the time the Constitution was written there were different ideas about what the phrase meant and competing legal theories about where the power to confer citizenship came from.

The meaning of the term is so unsettled that scores of constitutional experts have been writing about it in the weeks since Donald Trump made it an issue in the 2016 campaign.

I don't think Ted Cruz can even run for President until he can assure Republican voters that being born in Canada is not a problem. Doubt! — Donald J. Trump (@realDonaldTrump) January 19, 2016

To review, Ted Cruz was born in Canada in 1970, where his Cuban father was working at the time. But Cruz's mother was an American citizen, so under US immigration law, that made him an American citizen, too.

So he is citizen, yes, but does the Constitution require something more to be natural born? If not, why was the term there in the first place, instead of providing simply that a person had to be born a citizen?

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The simple answer is, it's impossible to know for certain. The founders devoted little time to discussing it. One day the term wasn't in the draft Constitution. The next day it was, and that was just about that.

The emerging consensus of the legal experts, however, is that being "natural born" means becoming a citizen at the moment of birth, as opposed to achieving it later through the process of naturalization.

"Natural in natural born doesn't mean biological. It means naturally, that is automatically, happening without any further intervention," said Constitutional law Professor Jack Balkin of Yale Law School.

Related: Cruz on White House Eligibility: It's Settled Law

And what could confer citizenship at birth? Clearly, being born in the United States, a concept familiar to the founders, who were well aware that anyone born in a place under control of the crown was a natural born British subject.

But the founders also knew that the British parliament could declare someone born abroad to British parents a "natural born subject."

The Constitution gave Congress that same authority through the power of naturalization, said Professor Michael Ramsey, of the University of San Diego School of Law.

Like the power British Parliament had, the congressional authority "includes both the power to establish rules for naturalizing foreign citizens on an individual basis and the power to declare categories of persons citizens by circumstances of their birth."

By the time Cruz was born, U.S. immigration law granted birthright citizenship to the child born overseas to at least one American citizen parent who had been living in the United States for a designated time.

Other experts, however, have said it’s not so clear cut. Professor Laurence Tribe of Harvard noted that, when the Constitution was written, citizenship passed through the father.

"Having just an American mother, as Cruz did, would clearly have been insufficient."

Related:Is Ted Cruz a Natural Born Citizen? Ask the Founders

Trump has repeatedly said Cruz himself could get this issue settled simply by going to court and asking for a decision.

But Trump is wrong about that.

"Cruz can't just go to federal court for a declaratory injunction without some plausible claim that he's in danger of government action being taken against him," said Yale’s Balkin.

One lawsuit has already been filed over the issue, by a retired federal prosecutor in Texas who claims to be suing on behalf of all registered voters. It will almost certainly be thrown out. The courts don't normally permit lawsuits claiming such a diffuse and abstract claim of injury.

But if a county or state official somewhere refused to put Cruz's name on the ballot, concerned he's not natural born, then Cruz could sue, and the issue would be squarely before the courts.

Federal judges then would be required to delve into the same history and struggle with the same issues that have animated legal experts over the Trump-Cruz controversy.

They will confront, said San Diego's Ramsey, a clause of the Constitution that is "mysterious and ambiguous."

And anything but settled.