The president is, as George W. Bush put it awkwardly but correctly, The Decider, and it is a fundamental rule of the job that the chief executive often has to decide among unpleasant options. President Obama explained this to Michael Lewis:

“Nothing comes to my desk that is perfectly solvable,” Obama said at one point. “Otherwise, someone else would have solved it. So you wind up dealing with probabilities. Any given decision you make you’ll wind up with a 30 to 40 percent chance that it isn’t going to work.”

Eight years ago I wrote about the job description of a president. One of my sources was Robert Caro, the two-time Pulitzer Prize-winning biographer of Lyndon Johnson. He described the disconnect between a presidential campaign and the actual duties of a president:

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“There’s endless months of debating about this job and almost no public discussion of what the job is. . . . There’s no other job like it. I’m sitting here watching Lyndon Johnson grapple simultaneously with riots in the streets, budget problems in Congress, are the Chinese going to come into Vietnam, what’s going wrong with the model cities program, how are we going to get the funding for Head Start, what’s Bobby Kennedy doing today, how are we going to blunt what he’s saying?”

No doubt Obama prezsplained much of that to Trump when they met Thursday. The session stretched to 90 minutes, and Trump said afterward that some of what he heard was “difficult.” I doubt that Obama told Trump that we are secretly enslaved to reptilian alien overlords from the Pleiades. More likely he told him what it’s like to be president day to day — the good and the bad. And he would have explained the limitations and constraints. Remember what President Harry S. Truman said when Dwight Eisenhower was seeking the presidency: “He’ll sit here, and he’ll say, ‘Do this! Do that!’ And nothing will happen. Poor Ike — it won’t be a bit like the Army. He’ll find it very frustrating.”

[Update: The Wall Street Journal reports, “During their private White House meeting on Thursday, Mr. Obama walked his successor through the duties of running the country, and Mr. Trump seemed surprised by the scope, said people familiar with the meeting. Trump aides were described by those people as unaware that the entire presidential staff working in the West Wing had to be replaced at the end of Mr. Obama’s term."]

Leon Panetta, who served as Bill Clinton’s chief of staff, told me eight years ago: “Running a campaign is one thing. Running around and doing events and shaking hands is all part of a campaign. But I think the reality is that we really don’t know what a person is going to be like as president of the United States until they walk into the Oval Office … And frankly, they probably don’t know as well.”

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Presidents can get lost in the weeds. That notoriously happened to Jimmy Carter, who got immersed in minutiae, such as the cotton dust standards imposed by federal safety regulators. You have to be able to delegate.

“If the job consists of mastering every tidbit, then it is too big for any one person. The last one that tried to do that was Jimmy Carter, and he got bogged down, because he was all trees and no forest,” Princeton professor Fred Greenstein told me.

“The other part of it, which is equally challenging, is to retain your humanity,” Panetta said. “The greatest danger a president faces is isolation from the very people who elected him.”

Trump is a creature of habit and he must now endure a major lifestyle change. He will have to live in a very old house that is essentially a rental. He will be surrounded by staffers and Secret Service agents and reporters and tourists and all these other people who might in some nightmarish situation try to shake his hand, slathering him with germs (he’s a germaphobe — check out the expression in that photo!). That’s not how he lives at Trump Tower. Perhaps he won’t even move to Washington. The man may decide to phone it in.

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Bob Woodward has described some of the intelligence briefings that the president-elect will get in coming weeks, including ones governing the nuclear codes:

The “football” also contains a book of options benignly called the “Presidential Decision Handbook.” This top secret/code-word book, known as the “Black Book,” of about 75 pages has separate contingency plans for using nuclear weapons against potential adversaries such as Russia and China. The president can select nuclear strike packages against three categories — military targets, war-supporting or economic targets and leadership targets. There are sub-options, and the menu allows a president to withhold attacks on specific targets. Two officials said that the “Black Book” also includes estimates on the number of casualties for each of the main options that run into the millions, and in some cases over 100 million. Officials who have dealt with nuclear-war options said that learning the details can be horrifying and that there is a “Dr. Strangelove” feel to the whole enterprise.

Problem: Trump doesn’t read books, even 78-page books. He made that clear in interviews with The Post. Here’s Marc Fisher reporting on Trump’s non-reading habits:

Trump said reading long documents is a waste of time because he absorbs the gist of an issue very quickly. “I’m a very efficient guy,” he said. “Now, I could also do it verbally, which is fine. I’d always rather have — I want it short. There’s no reason to do hundreds of pages because I know exactly what it is.”

I used to think of Trump’s candidacy as a bit like Mars One. That’s the Dutch reality TV show. Contestants compete for the chance to fly to Mars and live the rest of their life there. Thousands of people signed up to be candidates for the show/mission. But even in the unlikely event that the show producers could scrape together the money and rockets to send people to Mars, the technologies to sustain a Mars colony didn’t exist, according to M.I.T. engineers who studied the Mars One plan. So it didn’t matter what people believed: It couldn’t be done. Impossible. Because of physics, chemistry, biology. And I would have said that Trump couldn’t be president for all the reasons that people said over the past year that this could not happen — personality, political reality, the electoral map. He’d never held elective office! He barely had a campaign staff! He picked fights with people on Twitter at 3 in the morning!

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He was not presidential.

And here’s the stunning truth: The voters who went to the polls Nov. 8 largely shared that dim view of Trump. As Chris Cillizza has pointed out, in exit polls, only 38 percent of voters said they view Trump as qualified, only 35 percent said he has the temperament to serve effectively as president, and only 1 in 3 called him honest and trustworthy.