In December, two federal judges reached opposite conclusions on the constitutionality of the National Security Agency’s program for collecting and searching data on every phone call made in the United States. The first judge, Richard Leon in Washington, DC, deemed the program likely to be unconstitutional; the other, William Pauley of New York, deemed it perfectly lawful. What divided the two was the digital age itself.

Judge Pauley concluded that a 1979 analog-era Supreme Court case, Smith v. Maryland, which allowed the government to collect the phone numbers a person has called over a short period, required him to uphold the NSA program, which collects such phone data on every American’s every phone call, stores them in a massive database for five years, and uses sophisticated computers to search them for associations and networks without a warrant. Judge Leon, by contrast, concluded that the digital age fundamentally alters the kind of private information the government can glean from such records, and therefore the 1979 case does not govern. Leon is right: if we rigidly apply analog-era precedents to digital-era problems, the Fourth Amendment will no longer serve its intended purpose of protecting privacy from prying government eyes.

At first blush, Judge Pauley has a point. The Supreme Court in Smith reasoned that you have no expectation of privacy in the phone numbers you call because you share them with the phone company for billing purposes, and therefore the Fourth Amendment is not implicated by government collection of that information. If that is so, Pauley reasoned, why should the fact that the government is collecting those data on every American, combining them with everyone else’s data and searching them, change the equation?

But it does. The government made a similar argument two years ago in United States v. Jones, which addressed whether police use of a GPS to monitor a car’s travel on public roads around the clock for a month implicated the Fourth Amendment. The government—relying on another analog-era precedent upholding the use of a radio transmitter beeper to help track a car from a chemical distributor to a country house—argued that no privacy concerns were raised. In United States v. Knotts, the Supreme Court had reasoned that the driver had no expectation of privacy, given that he was traveling on a public road from point A to point B, because anyone could watch him do so. The government argued that the GPS used to track Jones’s car was just a more sophisticated beeper gathering the same public information.

But the government lost Jones, 9–0. The justices had different justifications, but five concluded that while we do not reasonably expect our travel from point A to point B to be private, we do have a reasonable expectation that we will not be followed 24/7 for a month, which would have been much more difficult for the police to do at the time of the Constitution’s framing, and which reveals much more than a single trip.

Judge Leon similarly reasoned that although we know that the phone company will record the numbers we call, for billing purposes, we do have a reasonable expectation that it will not save those records for five years, collate them with those of all other phone companies and search them to identify networks of associations. As in Jones, the computer-aided accumulation of data can reveal a great deal more about one’s private life than a single collection of data.

The broader question the NSA program raises is whether Fourth Amendment doctrine needs to be revised to account for the digital age’s exponential increase in the government’s ability to track us through information shared with third parties. In the old days, if the government wanted to know who all your friends were, what books you were reading and what you fantasized about, it would have to expend vast amounts of time and energy to follow you, and get a warrant to search your home—and it still would not be able to discern, for example, what you were thinking on any given day.

Today, virtually everything we do leaves a digital trace with some “third party”—our phone company, Internet service provider, e-mail server, credit card company, the bank, or the stores and online services where we shop. These companies know where we are, with whom we communicate and spend our time, what we buy and even what we’re thinking. Should this mean the government can collect and analyze all these data, on the analog-era reasoning that you have no expectation of privacy regarding what you share with a third party? If so, the government could collect not just our phone records but our location data, Internet browsing patterns and purchase histories—without any individualized suspicion or court supervision. Our lives would be an open book, available to government agents whenever they chose, even if we never engaged in suspicious behavior.

If that prospect is disturbing, then Fourth Amendment doctrine has to be revised. It’s been done before. It evolved to address the invention of cars, phones, thermal-imaging devices and GPS, to name just a few. It must evolve again if we are to ensure that the privacy protections the founders deemed essential to a free and vibrant democracy are not eliminated by technological default.