It's knowing what one doesn't know -- and knowing enough to surround oneself with those who do -- that makes for a smart president.
This is what the last Bush who served as president said today about the next Bush who wants to serve as president in the state where every Bush who's run has won a Republican primary election.
Which reminds us of what one of the chief advisers to the last Bush said:
"There are known knowns. There are things we know we know. We also know there are known unknowns. That is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know."
In a race in which the front-runner for the Republican presidential nomination has said he looks to a lot of experts on television for his understanding of foreign policy, the words of the former secretary of defense, Donald Rumsfeld, speak directly to a question before the voters of South Carolina, where George W. Bush campaigned today for his "big little brother," Jeb Bush.
"I know what I don't know,'' John Ellis Bush said during his own remarks.
The former president spent almost as much time during his 22 minutes on stage today reminding voters why they should be wary of Donald Trump in Saturday's Republican primary as he did explaining why brother Jeb is the one to choose.
"In my experience, the strongest person usually isn't the loudest in the room," the former president said campaigning for his brother in North Charleston. "We do not need someone in the Oval Office who mirrors and inflames our frustration."
"Strength is not empty rhetoric,'' he said. "It is not bluster.''
If the younger Bush has always chafed at characterizations of him as "the smarter one'' -- or "the more cerebral one'' in recent press narratives -- the older one was characteristically self-effacing today in noting that he's written a couple of books since leaving office, confounding those people "up East" who didn't think he could even read, or that the value of his retirement paintings rests more in the signature than the material.
"I've been mis-underestimated most of my life,'' Bush joked.
Yet, he said, he understood clearly the job at hand as he listened to a classroom of Second-graders reading to him in Sarasota, Florida, on the morning of Sept. 11, 2001.
"I became something that no president should ever want to be, a war-time president,'' he said.
And that's when the people with whom Bush had surrounded himself became so important.
During his own campaign for president, Jeb Bush initially had trouble explaining if he would have done the same thing his brother had done in Iraq, knowing what we know now.
"The ones we don't know we don't know,'' per Rumsfeld.
In this case, Trump is the "known unknown.''
Yet Jeb Bush has faced his own troubles becoming known to voters nationally.
The younger Bush was widely criticized early in his campaign for saying he'd call on some of the same people his brother had called upon for the advice he'd need as president.
Jeb Bush, George W. Bush said today, is "going to assemble a great group of people.''
Now, in the closing days of one state's primary campaign in which Jeb Bush is struggling for the No. 3 finish in the third contest of the political season, the Bushes are asking South Carolinians to stop and consider what they don't know about the party's front-runner, and what they should know about the 45th president.
They're asking voters to trust a leader to understand that which he doesn't know and to surround himself with those who do.