“Do I contradict myself? Very well, then I contradict myself. I am large. I contain multitudes.”- Walt Whitman, “Song of Myself”

We understand, Walt. We’re complex characters, we humans.

And sometimes we change our minds.

We vote for Democrats, then switch to Republicans. We change our minds about jobs, houses, spouses. And with each new shift in thinking come chances to make new leaps in our lives.

Big deal? It can be. As Norman Vincent Peale, author of “The Power of Positive Thinking,” said. “Change your thoughts and you change your world.”

As we write this, millions of people are changing their minds about subjects ranging from immigration and global warming to what presidential candidate they support.

It’s not so much thinking as re-thinking. And unless we believe ourselves infallible, such critical re-evaluation is an essential part of being human. It’s what Socrates was getting at when he said the unexamined life is not worth living.

While it’s easy to change your mind about trivial matters, doing so about important subjects is often difficult and uncomfortable. Before we can adopt a new belief, we must admit an old one was wrong. And if that belief was wrong, what others may need changing?

Minds may change slowly on serious issues. But when enough minds change, the result can be dramatic.

Take this year’s Democratic voters. They’re on the verge of making one of the most historic leaps in political history. After more than 200 years of nothing but white male presidential nominees, this year’s Democratic front-runners are a woman (Hillary Clinton) and an African-American (Barack Obama). Considering that women couldn’t vote before 1920 and that America once enslaved black people, that’s a lot of changed minds.

Don Haider-Markel, associate professor of political science at the University of Kansas, said the biggest change in voters’ minds came 15 years ago when the number who said they’d consider supporting a qualified minority or female candidate for president climbed above the critical 75 percent mark in national polls. Obama and Clinton are the first candidates to tap into the new mindset of a changed electorate.

But then, changing your mind is a funny thing. While voters think it’s fine to change their minds about politicians, they don’t have a stomach for politicians who change their minds about issues. Just ask former Democratic presidential candidate John Kerry, who was ridiculed by opponents as a waffling flip-flopper for saying he actually did vote for the war before he voted against it.

“Voters have a level of uncomfortableness with ambivalence,” Haider-Markel said. “We expect our politicians to have a certain amount of certitude about their positions.”

Then again, we don’t like them to be too sure.

“The very thing that many voters liked about (President George W.) Bush – his certitude and unwillingness to change -became the same thing he was criticized for after taking office,” he said.

Changing your mind is easier if you’re a scientist or a doctor. You change only when the facts dictate it. Scientists once thought the Earth was flat and, more recently, that Pluto was a full-size planet.

Oops.

And for more than 100 years doctors thought that stress and spicy food led to peptic ulcers. Thanks to new research, they now know the real culprit is a bacterial infection.

For anyone to grow, educators say, they must first be open to change.

Marti Smye is a Miami expert in change management who helps executives make important changes.

“It’s hard to change your mind,” Smye said. “Some people think it makes them look indecisive. For others it is all ego. But the best leaders don’t have as much trouble changing their mind. No. 1, if you’re comfortable in your own skin and you feel your decision-making is objectively based, you should be open to new data and feedback. If you’re closed to feedback, you’re going to be very reticent to change your mind. You know, “I don’t want to hear it. I’ve got my mind made up.”‘

That may be consistent, but it’s not how American society works. We change our collective mind all the time. We once thought it was impossible to put a man on the moon, or tear down the Berlin Wall. John Kennedy and Ronald Reagan helped us change our minds about those.

And look at movies. Films such as “Citizen Kane,” “It’s a Wonderful Life” and “The Wizard of Oz” didn’t open to universal praise and blockbuster business. It was only after many years of reflection, critics say, that people began to recognize them as classics.

Some recent turnabouts

So what are people changing their minds about today?

Kevin Kelly, editor at large at Wired magazine, said – after initial skepticism – he has changed his mind about Wikipedia, the free online encyclopedia that anyone can edit. He now praises it, saying, “It keeps getting better.”

Recently the Web site slashdot.org posted a question for discussion: “What did you change your mind about in 2007?”

Actor Alan Alda wrote that he changed his mind about God. Twice. First he believed, then he became an atheist. Today, he’s an agnostic. In the same way no one could prove there was a God, he wrote, no one could prove there wasn’t.

Stewart Brand, founder of the Whole Earth Catalog and a man the Los Angeles Times described as “the least recognized most influential thinker in America,” changed his mind about the value of old things. After pining for old wooden sailboats, he finally bought a few.

“Their gaff rigs couldn’t sail to windward,” Brand wrote. “Their leaky wood hulls and decks were a maintenance nightmare. I learned that the fiberglass hulls we’d all sneered at were superior in every way to wood.”

He discovered the same thing applied to windows, doors and kitchen cabinetry when he remodeled an old farmhouse.

Not everyone is open to change, said Wayne Vaught, associate professor of philosophy at the University of Missouri-Kansas City.

“A lot of people believe there are absolute truths that are uncontestable, and as soon you discover (them) there’s no reason to engage in any more inquiry.”

But after being exposed to new ways of critical thinking, many of Vaught’s students change their minds about a range of issues, from euthanasia to capital punishment.

“I’d be somewhat disappointed if they didn’t,” he said.

“What people do is think critically about their lives,” he said. “Hopefully during that critical reflection, reasons will be given for accepting one belief over the other and potentially changing one’s mind.”

If you never changed your mind, you’d never try new products or new foods. You’d never take a risk. What’s more, America as we know it would be in big trouble. Our economy and our political system depend on a steady flow of consumers and voters changing their minds as new products appear and new challenges arise.

Even dating and love depend on being able to change someone’s mind.

The point? The world is constantly evolving, and so are we. As we grow, we mature and change. We learn new things, meet new people, and – sometimes – we change our minds.

The only way to ensure you will never change your mind?

Stop thinking and stop caring.

Good luck with that.