Someday a stranger will read your e-mail, rummage through your instant messages without your permission or scan the Web sites you’ve visited — maybe even find out that you read this story.

You might be spied in a lingerie store by a secret camera or traced using a computer chip in your car, your clothes or your skin.

Perhaps someone will casually glance through your credit card purchases or cell phone bills, or a political consultant might select you for special attention based on personal data purchased from a vendor.

In fact, it’s likely some of these things have already happened to you.

Who would watch you without your permission? It might be a spouse, a girlfriend, a marketing company, a boss, a cop or a criminal. Whoever it is, they will see you in a way you never intended to be seen — the 21st century equivalent of being caught naked.

Video: Online privacy woes Psychologists tell us boundaries are healthy, that it’s important to reveal yourself to friends, family and lovers in stages, at appropriate times. But few boundaries remain. The digital bread crumbs you leave everywhere make it easy for strangers to reconstruct who you are, where you are and what you like. In some cases, a simple Google search can reveal what you think. Like it or not, increasingly we live in a world where you simply cannot keep a secret.

The key question is: Does that matter?

For many Americans, the answer apparently is “no.”

When pollsters ask Americans about privacy, most say they are concerned about losing it. An MSNBC.com survey, which will be covered in detail on Tuesday, found an overwhelming pessimism about privacy, with 60 percent of respondents saying they feel their privacy is “slipping away, and that bothers me.”

People do and don't care

But people say one thing and do another.

Only a tiny fraction of Americans – 7 percent, according to a recent survey by The Ponemon Institute – change any behaviors in an effort to preserve their privacy. Few people turn down a discount at toll booths to avoid using the EZ-Pass system that can track automobile movements.

And few turn down supermarket loyalty cards. Carnegie Mellon privacy economist Alessandro Acquisti has run a series of tests that reveal people will surrender personal information like Social Security numbers just to get their hands on a measly 50-cents-off coupon.

But woe to the organization that loses a laptop computer containing personal information.

When the Veterans Administration lost a laptop with 26.5 million Social Security numbers on it, the agency felt the lash of righteous indignation from the public and lawmakers alike. So, too, did ChoicePoint, LexisNexis, Bank of America, and other firms that reported in the preceding months that millions of identities had been placed at risk by the loss or theft of personal data

So privacy does matter – at least sometimes. But it’s like health: When you have it, you don’t notice it. Only when it’s gone do you wish you’d done more to protect it.

But protect what? Privacy is an elusive concept. One person’s privacy is another person’s suppression of free speech and another person’s attack on free enterprise and marketing – distinctions we will explore in detail on Wednesday, when comparing privacy in Europe and the United States.

Still, privacy is much more than an academic free speech debate. The word does not appear in the U.S. Constitution, yet the topic spawns endless constitutional arguments. And it is a wide-ranging subject, as much about terrorism as it is about junk mail. Consider the recent headlines that have dealt with just a few of its many aspects:

Hewlett Packard executives hiring private investigators to spy on employees and journalists.

Rep. Mark Foley sending innuendo-laden instant messages – a reminder that digital communication lasts forever and that anonymous sources can be unmasked by clever bloggers from just a few electronic clues.

The federal government allegedly compiling a database of telephone numbers dialed by Americans, and eavesdropping on U.S. callers dialing international calls without obtaining court orders.

Privacy will remain in the headlines in the months to come, as states implement the federal government’s Real ID Act, which will effectively create a national identification program by requiring new high-tech standards for driver’s licenses and ID cards. We'll examine the implications of this new technological pressure point on privacy on Thursday.

What is privacy?

Most Americans struggle when asked to define privacy. More than 6,500 MSNBC readers tried to do it in our survey. The nearest thing to consensus was this sentiment, appropriately offered by an anonymous reader: “Privacy is to be left alone.”

The phrase echoes a famous line penned in 1890 by soon-to-be Supreme Court Justice William Brandeis, the father of the American privacy movement and author of “The Right to Privacy.”At the time, however, Brandeis’ concern was tabloid journalism rather than Internet cookies, surveillance cameras, no-fly lists and Amazon book suggestions.

As privacy threats multiply, defending this right to be left alone becomes more challenging. How do you know when you are left alone enough? How do you say when it’s been taken? How do you measure what’s lost? What is the real cost to a person whose Social Security number is in a data-storage device left in the back seat of a taxi?

Perhaps a more important question, Acquisti says, is how do consumers measure the consequences of their privacy choices?

In a standard business transaction, consumers trade money for goods or services. The costs and the benefits are clear. But add privacy to the transaction, and there is really no way to perform a cost-benefit analysis.

If a company offers $1 off a gallon of milk in exchange for a name, address, and phone number, how is the privacy equation calculated? The benefit of surrendering the data is clear, but what is the cost? It might be nothing. It might be an increase in junk mail. It might be identity theft if a hacker steals the data. Or it might end up being the turning point in a divorce case. Did you buy milk for your lactose-intolerant child? Perhaps you’re an unfit mother or father.