“Teachable.”

That’s the adjective you hear from John McCain’s advisers. The economy may not be the Republican front-runner’s favorite topic. But the man is “open” and “capable of learning.” With the right teaching, Senator McCain could be another Ronald Reagan.

This argument is laughable. On economics, Mr. McCain is about as impressionable as the statue of General Ulysses Grant on the National Mall.

To Mr. McCain, tax proposals aren’t teachables. They are tradeables, something you give up in exchange for concessions from colleagues in other areas that matter more to Mr. McCain. No wonder talk-radio dragons have so much fun scorching the senator.

But Mr. McCain doesn’t have to be teachable to be worthy of endorsement. Even endorsements by Reaganauts.

The best way to explain why Mr. McCain can’t be Reagan is to consider the nature of the presidency. Within hours of their inauguration, presidents find that the pressure of events gets to them. The mammal brain — the glad-handing part —and the lizard brain — the part that will do anything to survive — take over.

Sure, presidents use that uniquely human section, the cerebral part where health care proposals live. But even that brain relies heavily on concepts and facts it took in at a younger stage, when the brain’s owner was in, say, his 20s. You don’t have to be a pop psychologist to see the pattern. It holds with most presidents, whether they are 71, Mr. McCain’s age, or 46, the age of Senator Obama.

Well, we know what filled Mr. McCain’s brain in his 20s. The waters of Truc Bac Lake flowing into his plane, withstanding torture, years in isolation pondering his wars and his father’s. It is safe to say that during his time at the Hanoi Hilton, Mr. McCain didn’t follow the congressional debate on the extension of the 5% income surtax.

The things that preoccupied the younger Reagan were entirely different — 53 film contracts, to be precise. While playing all those roles and after, Ron had plenty of hours to worry about the Hollywood left, about the inefficiency of cartels (his Screen Actors Guild) and the disadvantages of high taxes on corporations (his period at General Electric Co.).

Reagan spent so many years wondering at the power of markets that he came to believe that broadening the U.S. tax base might weaken the bargaining position of Leonid Brezhnev. To Reagan, the Treasury Department worked for the Pentagon, and vice versa. Reagan’s was a great insight, but not everyone shares it.

If there is a recent president Mr. McCain resembles, it is not Reagan but Dwight Eisenhower. In 1960, a Republican, Eisenhower’s vice president and heir, campaigned on the GOP’s foreign-policy experience. The Democrat was a regular Barack Obama, a young senator from Massachusetts whom opponents tried to cast as callow. The Republican lost, which is why McCain advisers won’t like this analogy.

Mr. McCain is also like President George H.W. Bush, the current president’s father, a veteran diplomat and a man of character. Bush Sr.’s instincts took over even before Iraq’s invasion of Kuwait. He joined West German Chancellor Helmut Kohl in wagering that German reunification was all important, an opportunity to be snatched.

Mr. Bush, the veteran diplomat, was concentrating so hard on foreign policy projects that all it took was a few intimidating memos about deficits to make him forsake his tax promises. The memory of that betrayal was still strong in 2000. One reason George W. Bush of Texas beat Senator McCain for the presidential nomination in 2000 was that Republicans figured he would never repeat his father’s error. But there is a legitimate reason for Reagan Republicans to back Mr. McCain this time. It is that 2008 is different from 2000. Foreign policy is still the most important issue in the country, no matter what voters tell pollsters.

Just because Americans are tired of foreign wars doesn’t mean those wars will end. When it comes to foreign policy, Mr. McCain is more committed than Mitt Romney. In fact, Mr. McCain’s support of the current administration’s war policy, rather than his tax positions, is what has drawn many economists to support him. So here are some realistic presidential scenarios for John McCain.

The good one: Osama bin Laden captured; Iraq surge continues to work; President McCain signs bill reforming Social Security while still keeping the cap on pension payments.

The less good one: Bin Laden isn’t found; something horrible happens in Europe; President McCain signs Social Security reform that includes massive tax increase on the wealthy.

The least good scenario: a larger war than Iraq is declared. Housing recession widens deficit significantly. President McCain leads country in crucial military campaign. He gives up on campaign promise to make Bush tax cuts permanent and signs on to some egregious additional increases.

To many free marketeers, these scenarios — even the last one — are preferable to the one in which a big war is lost. Especially if Mr. McCain names someone they respect — former Senator Gramm, a McCain supporter, for example — to head the Treasury.

After all, the first President Bush and certainly Mr. Kohl believed that the feats of German reunification and Desert Storm redeemed whatever flaws there were in their economic policies. And many recognize that they were right.

The agony for the Reaganauts is that some know that those economic sacrifices aren’t necessary, that there isn’t always a tax-war trade off. And they are right, too.

Mr. McCain is no Reagan, but he’s still a pretty good Republican candidate. At this point, the spinmeisters can offer up any description they like of the man.

Just don’t say he’s teachable.

Miss Shlaes, a senior fellow in economic history at the Council on Foreign Relations, is a columnist for Bloomberg News.