No sooner had Google Inc. yielded to popular pressure to bar facial-recognition applications from Google Glass than techies split into two factions: those who called the ban an outrage that would hurt law enforcement and medical care, and those who said the ban would make no difference because sooner or later the wall was bound to fall.

The original concern rested on the notion that wearable facial-recognition technology would constitute a threat to privacy -- the privacy, for example, of those who would prefer to walk the streets unrecognized. Google’s position continues to be that privacy concerns about Glass are overblown. I think the long run will prove Google right, for reasons less technological than generational.

Glass is essentially eyewear connected to a processor and the Internet, with an interior display that looks to the wearer like a television screen. This sounds so cool and space-agey that it’s at first hard to see how the privacy issues arise.

Certainly the claim that the famous have a “right” to wander the streets incognito seems thin, and Google initially seemed inclined to ignore it, preferring to tout the obvious utility for, say, police officers able to identify suspects at a glance. The company might never have done its about-face had not the disclosure of the scope of National Security Agency’s data-mining program (and Google’s own unclear participation therein) brought privacy to the forefront of public conversation.

But there are other privacy issues. For example, Nick Bilton of the New York Times was startled to encounter Google Glass wearers in the men’s room, immediately after being reminded that “one of the gadget’s greatest features is the ability to snap a photo with a wink.”

Creepy Glasses

Creepy indeed. But privacy as traditionally understood encompasses far more than photographs in the bathroom or recognition on the street. Privacy is fundamental to life in liberal society, where, as the philosopher Judith Shklar argued, our fear of abuse of authority should lead us to establish constraints on both government and private power -- so that we are free, in almost everything we do, to make our choices without worrying about retaliation.

Ten years ago, in a report titled “Bigger Monster, Weaker Chains: The Growth of an American Surveillance Society,” the American Civil Liberties Union gave an example of privacy in this traditional sense: “A woman who leaves her house, drives to a store, meets a friend for coffee, visits a museum, and then returns home may be in public all day, but her life is still private in that she is the only one who has an overall view of how she spent her day. In America, she does not expect that her activities are being watched or tracked in any systematic way -- she expects to be left alone.”

That familiar cultural understanding has been upended -- not all at once by post-Sept. 11 anti-terror measures, but gradually, as more and more of our lives are lived out digitally. The woman in the ACLU’s example might use GPS, thus revealing her location as she drives. Perhaps she talks on her mobile phone, stops to buy gas or pays for coffee with a credit card. All of these activities leave digital traces. To imagine that those traces simply vanish, unrecorded, is no longer reasonable.

And reasonableness is very much the word to think about. When we conceptualize privacy as a right, our thoughts turn naturally to the Constitution. According to the judges, the measure of what is private turns on what a reasonable person would expect to be private. But the reasonable person is a shifting target, her expectations informed by culture and mores and actual practice.

Once everybody knows what so many already suspected -- that the federal government eavesdrops extensively on the people it purports to represent -- our expectations suddenly shift. One might be a passionate opponent of the programs, or one might accept their grim necessity; either way, the reality of their existence necessarily changes the way we think.

Cool Glasses

How much has the digital world changed the conception of privacy? A recent study by the Annenberg Center for the Digital Future at the University of Southern California concluded that the so-called millennials -- those ages 18-34 -- don’t share the fears of their elders about online privacy. In particular, the rising generation seems willing to trade privacy for other gains. They are, for example, “more receptive than older users to accepting targeted advertising when their personal information is required.”

Facebook founder Mark Zuckerberg has been widely criticized for making essentially this very point, but the data suggest he is right. The millennials do indeed tend to have a different view of where one draws the privacy border. As a headline several years ago in New York magazine declared: “The future belongs to the uninhibited.”

A recent Washington Post-Pew Research Center survey purports to show that those ages 18 to 39 are slightly more worried than their elders about the NSA program. But the same poll indicates that this group is far less likely to have paid close attention to the story. The episode seems to be rolling off their backs, perhaps because the young always assumed that somebody was cataloguing their telephone calls, Internet searches and credit-card transactions.

According to the companies that market to them, millennials are quite sophisticated in using privacy controls to limit access to their online data. But they seem to worry a lot more about keeping their online activities from parents and teachers than from corporations and government.

The point is that the reasonable person and her expectation of privacy are reconstructed in every generation. Criticizing Google for supplying what is certain to be heavy demand for so cool an advance as Glass is no way to hold a serious conversation. And a serious conversation is what will be needed -- not among us Baby Boomers, but among those who follow us.

That will be an intriguing challenge. It’s important for members of the rising generation to be active in shaping their expectation of privacy, and so demand law and practices that strike a reasonable balance. The alternative is to drift along, allowing their expectations to continue to be shaped for them, by the twin determinisms of technology and security.

(Stephen L. Carter is a Bloomberg View columnist and a professor of law at Yale University. He is the author of “The Violence of Peace: America’s Wars in the Age of Obama,” and the novel “The Impeachment of Abraham Lincoln.”)

To contact the writer of this article: Stephen L. Carter at stephen.carter@yale.edu or @StepCarter on Twitter.

To contact the editor responsible for this article: Michael Newman at mnewman43@bloomberg.net.