Stupidity, I’ve heard it said, is defined as continuing to do the same thing, but expecting a different result. In this, stupidity is different from simple ignorance.

Let’s say, for instance, that you have no idea how it feels to bash your thumbnail with a hammer. So you do that and discover it to be a tremendously unpleasant sensation. That’s an act of ignorance; you didn’t have the information.

Now, let’s say you bash your thumb again.

That’s an act of stupidity; you had the information but were unable or unwilling to process it, follow it to its logical conclusion.

Forty years later, the United States’ embargo against Cuba feels a lot like that. Through nine U.S. presidencies, Kennedy to Johnson to Nixon to Ford to Carter to Reagan to Bush to Clinton to Bush, we have embraced it as a means of pressuring Fidel Castro’s communist dictatorship toward democratic reform. Or toppling it altogether.

If we’ve made any progress toward either goal, I must have missed it. If there’s the scantest reason to believe change is coming anytime in the near future, I must have missed that, too. Yet we cling to our policy with reflexive stubbornness.

You saw this in the White House response to President Jimmy Carter’s historic Tuesday night speech in Havana, where he called for lifting the trade embargo. That call was promptly echoed by a bipartisan group of legislators.

Not going to happen, replied the Bush administration. Said White House spokesman Ari Fleischer, “The president believes that the trade embargo is a vital part of America’s foreign policy.”

Excuse me, but … why? On what basis can the president’s belief possibly be sustained?

Certainly, it’s difficult to see where the embargo has yielded any strategic benefits for the United States. Lately, even its political benefits are somewhat less than certain. During the Cold War, of course, support for the embargo was a litmus test of sorts. No lawmaker wanted to open himself or herself to the charge of being “soft” on communism. It has also been observed that any president or candidate who was less than enthusiastic about the embargo risked alienating a vital voting bloc, South Florida’s Cuban exile community.

Not that political expediency justifies failure to do the right thing, but the question is moot in any case. The Cold War is over. And the exile community’s support for the embargo is anything but monolithic, as illustrated in a poll, conducted in April, and reported this week by the Miami Herald. The survey, by the Miami firm of Bendixen & Associates, found that though 61 percent of the exile community want the embargo to continue, 52 percent believe it should no longer be the spear point of U.S. policy toward the Castro government and ought to be replaced by other measures. Perhaps more significantly, nearly half of those polled reported sending money to their relatives in Cuba, transactions that are said to pump as much $950 million a year into that nation’s economy.

Which would, at a minimum, appear to undermine both the spirit and efficacy of the embargo. So again: Why?

The issue is not whether the Castro regime is a moral monstrosity. It is. But so was South Africa under apartheid. So are China and Saudi Arabia now. Yet somehow, we’ve found ways to do business with all of them. Indeed, we’ve used our relationships with those nations to nudge them toward human rights reform.

No, that has not proved to be a perfect solution, either. But at least there is movement, at least there is give and take, at least there is an arena in which change could conceivably ferment.

We have none of those with Cuba. Instead, we apparently embargo primarily because we’ve always embargoed and can’t figure out how not to. We punish the Cuban people, punish our own exporters and for what? The thing hasn’t worked. Shows no prospect that it ever will.

We’ve spent 40 years doing the same thing and expecting a different result. Apparently, we’re ready to spend 40 more.

We can no longer claim ignorance. And you know what’s left.

Leonard Pitts Jr. is a columnist for the Miami Herald.