“Pride cometh before a fall.” It’s an Old Testament proverb, it’s the classic theme of Greek tragedy, and it’s a lesson we observe in our lives again and again. So how come we never seem to learn to avoid it — or exploit it?

What’s more, pride always seems to come before a fall in more or less the same way.

Take the economy, for example, and consider three things pride does — and the havoc that follows.

First, pride makes us forget first principles. In the 21st century economy, we got so used to having more and more money that we forgot that it’s necessary to save. Your mortgage is supposed to be a certain percentage of your income. You aren’t supposed to live beyond your means.

But as the economy grew more and more, we began to say, “It’s a good thing we live in a time when we don’t have to follow all those old rules.” When it turned out we weren’t exempt after all, it was too late.

The second thing pride does: It makes us lose our vigilance. We forget to be on the lookout for the telltale signs of disaster ahead.

With the economy, we saw the dot-com bubble burst and didn’t change our behavior. We watched the housing market collapse and didn’t change our behavior. Then we saw the mortgage crisis hit as gas prices skyrocketed and we started to sound the alarm, but not loudly, and not in time.

A third effect of pride: We use our imagination to show off instead of to build. We stop elaborating big plans and start planning big elaborations.

Our economy was built by entrepreneurial imagination. Bold businesses took risks to create opportunities where none existed before. We created whole industries and new streams for old industries.

But with pride comes complacency, and complacency and imagination are a volatile brew. Our best business minds still found great new opportunities and new streams of revenue — but this time it was about maximizing and displaying wealth instead of building industries and creating opportunities.

Whether it’s the Lehman Brothers’ flashy Times Square Jumbotron that cost $500,000 to run, the infamous AIG excesses, or the extravagant expenditures revealed in the trials of CEOs, a lot of pride seemed to come before a lot of falls.

We can see this same pattern in Washington, D.C.

The GOP rose to power in Congress in 1994 on a set of principles enumerated in its Contract With America. The Republicans ended perks for members of Congress and their staffs. They enacted a balanced-budget amendment. They promoted the line-item veto for the president to curb pork-barrel spending.

But in the pride of their victory, they let those principles slip away, and they became the epitome of the excesses they were elected to oppose.

In the minority, the GOP had been vigilant about what people wanted and how they were being perceived. They knew their weaknesses as well as their strengths, and played a careful game.

But in the thrill of power, they forgot their weaknesses — and soon forgot their strengths, too.

In the years leading up to their victory, the GOP crafted clever ideas to better spend the nation’s welfare dollars, better structure the tax code to increase savings and investment, and even tried to address systemic problems in entitlements.

But once ensconced in Washington offices, their new ideas were all about new pork-barrel spending projects.

Today we have new leaders in Washington, and they are certainly proud of their accomplishments.

The election of Barack Obama was historic — but it was historic as a beginning, not as an end in itself. His supporters don’t seem to notice. The celebrations seem to take it for granted that Obama will be successful, that he will turn the economy around, that he will bring peace to our times.

Don’t count on it. What’s most likely to happen is what has happened so often before.

When John F. Kennedy won, there was a similar euphoria in the air — but the disastrous Bay of Pigs invasion sucked away all the excitement that lingered after Inauguration Day. And the same people who cheered Kennedy at the start of his presidency were not long after protesting the Vietnam War he launched.

When Bill Clinton was inaugurated, The Washington Post’s headline read: “A New Era.” But the new era didn’t last very long as Clinton’s fledgling administration entered a bruising battle over homosexuals in the military. Two years later, the GOP would sweep Congress.

It all reminds one of the premature “Mission Accomplished” sign on the aircraft carrier that George W. Bush visited.

What of Barack Obama? His victory celebrations began over the summer when he created his own presidential seal and held rock-concert-like appearances in Europe. He appeared in a temple at his convention. The euphoria reached near pathological intensity after his election.

What cometh after pride this time?

Washington’s new elite will surely push pro-abortion principles, hard. Let’s push back, even harder, and wait for them to let up.

The echo chamber of a Washington in which one party holds Congress and the White House will make it easy for anti-family forces to let their guard down. Let’s not let ours down.

And when the imagination of the Washington elite turns to self-aggrandizement, as it always does, let’s think of new ways to promote life.

In the battle ahead, those who stay humble and hungry just might have a chance.