" With this president, you never know."

That's a sentence I've heard several times as I've covered Donald Trump's first 100 days in the Oval Office—not from pundits or journalists or members of Congress, but from a high-ranking White House aide. That feeling of uncertainty permeates the West Wing and the entire administration, on everything from policy to personnel. Decisions are often made at the last minute, and the results are sloppy or half-finished. Aides find themselves scrambling to respond coherently to presidential tweets or promises he makes in interviews with the media. With several unfilled positions in the White House and departments and agencies, staff feel stretched thin. Lawmakers remain unsure about administration policies.

You never know—and that means President Trump has pleasantly surprised his critics at times. His nomination of Neil Gorsuch to the Supreme Court was a big win for conservatives worried about his fealty to his pre-election list of originalist judges. The strike on Syria following Bashar al-Assad's chemical-weapons attack was an abrupt change from the previous seven years of policy—and from Trump's own campaign rhetoric about the civil war there. On some sensitive issues, such as the Iranian pursuit of nuclear weapons, Trump has learned to be more careful and measured in the words he says. As his presidency continues, those words may hit even closer to their target.

You never know, because, simply put, Donald Trump is an ad hoc president. The decisions he makes are by and of the moment, with his aides and staffers and supporters racing to fit them into a message and a policy. Here's an example: When Trump commented in January, offhandedly and with no evidence, that 3 to 5 million illegal votes cost him a popular vote victory, and he further added he was calling for an investigation, his administration had to snap into action. The White House press secretary, Sean Spicer, defended the president's comments, while the communications office spent time unearthing small-scale evidence of limited illegal voting. In a press briefing, Spicer suggested there would be an executive-branch investigation. By February, Vice President Mike Pence had been tapped to lead it. But since then, there's no sign anyone in the administration is actually doing any investigating. The president, meanwhile, appears to have moved on. Instances like this meandering trip make following Trump's first 100 days feel like a mix between constant whiplash and a continuous nervous breakdown.

It's not unusual for a new administration to have problems with disarray or confusion. The beginning of a term is when the president is the strongest politically, but the weakest operationally. Staffing takes time and vetting, and political appointees have to learn to get along with career bureaucrats. The job of the president—the biggest job in the world—takes some getting used to.

But the Trump presidency is unique in how this early disorder is defined and shaped by the tendencies of the principal at the top. "Trump is the face of Trumpism," one senior White House official put it to me. As another senior administration official recently said of Trump's foreign policy doctrine, "It's not doctrinaire at all." After floating earlier this month that Trump had "scrapped" his campaign tax-reform plan and would be going "back to the drawing board," the administration this week rolled out the beginning of a new proposal that looked remarkably like what he had released during the campaign.

Compare that to another Republican president, Ronald Reagan. As the late Kate O'Beirne, a veteran of the Reagan administration, once said, "When you worked for Reagan, you just knew what to do. Nobody had to tell you what to do. You didn't wait for orders. You got up in the morning, went to your job, and did what you knew Reagan wanted done because you were a conservative." Reagan was clear in what he believed and what he wanted, and so were those who worked for him. There's no such clarity with Trump.

Trump is not driven by details, preferring to communicate in broad strokes and to govern by executive order. He's ambitious and slapdash with his policy goals, yet he will quickly abandon initiatives when they struggle. In areas about which Trump knows little, he's more than willing to cede major parts of the decision-making process to the experts he trusts (with mixed results). He's quick to distance himself from loyal aides when they become problematic. His administration becomes consumed with self-imposed distractions that do nothing to advance his agenda.

The Trump presidency so far has reflected the man himself, and for that reason, the next 1,360 or so days of his term in office are likely to be just as much of a roller coaster ride as the first 100.

But you never know.