Leading Republican presidential candidate Ben Carson has never held elected office and isn't known for his in-depth understanding of public policy issues. Critics have questioned whether it makes sense to elect him president. But Carson has a response to this criticism:

It is important to remember that amateurs built the Ark and it was the professionals that built the Titanic. @my_ccu pic.twitter.com/6Nqod4sicS — Dr. Ben Carson (@RealBenCarson) October 29, 2015

This is a common sentiment among Republican voters. Many conservatives believe that professional politicians and other Washington insiders have made a hash of things, resulting in soaring deficits, crony capitalism, and other ills. So there's a lot of appeal to the idea of the citizen politician — someone who makes his or her career in the private sector, spends a few years in government, and then goes back to private life.

Conservatives like to hark back to the early days of the American republic, when this kind of thing was more common. In the 19th century, getting elected to federal office didn't require raising millions of dollars, so it was easier for someone with no prior political experience to get involved in politics. Supporters of Ben Carson see him as the 21st-century embodiment of this ideal.

But the ideal of the citizen politician misses something important about how the United States has changed over the past two centuries. In the 19th century, it was easy to be an amateur politician, but it was also a lot easier to be an amateur lawyer, journalist, scientist, or physician. That's because the state of the art in all of these fields wasn't very good, so it didn't take very long to master them.

Today no one would want an amateur doctor to do brain surgery on them — they'd want a highly trained professional like Ben Carson. Most jobs doing scientific research require a PhD, because it takes years of study to master the scientific state of the art. And it's a good thing that, generally speaking, we expect politicians to have some previous experience in government — or related fields — before elevating them to the highest office in the land.

This doesn't necessarily mean people need to be a member of Congress or a governor before becoming president. Dwight Eisenhower, for example, became president after a successful career in the military.

But people do need relevant experience. Carly Fiorina, for example, spent six years managing one of the nation's largest companies — a job with many similarities to the presidency — and has spent the years since then advising the CIA. And it's clear from interviews that she's done her homework, mastering many of the complex policy issues she'd be called on to deal with as president.

Carson, by contrast, seems to relish his standing not only as an outsider but as a non-expert in public policy. He seems to believe that he'd be able to govern by applying "common sense" to the nation's problems. But this is nonsense. The presidency is a hard and complex job. Electing a true amateur to the White House makes as little sense as having an amateur doctor do brain surgery.