In a surprising decision, Chief Justice John Roberts joined the Supreme Court’s four liberals in holding that the government must obtain a warrant in order to search your cellphone location data. The decision is a huge win for privacy rights in the digital era, and signals a new phase in the Supreme Court’s protection of civil liberties in the 21st century.

It also makes one wonder if Chief Justice Roberts might become the Justice Kennedy of the next twenty years: voting conservatively on most issues but siding with liberals on others.

The facts of the case, Carpenter v. U.S., are simple: Police obtained the cellphone location data of a suspect in a robbery case and used it to convict him. The question was whether getting that data constitutes a “search” under the Fourth Amendment, which would require a warrant before conducting it.

As of today, the answer is yes.

The relevant precedent was so close, the answer was practically a judicial coin flip. On the one hand, the Court has ruled in the past that information you share with a third party (in this case, your cellular provider) is no longer private, and so no warrant is needed. That would indicate no warrant is needed here.

On the other hand, we live in a new world in which cellphones are “almost a feature of human anatomy,” in the words of one prior case, and the location sensing is necessary for them to work. Should the government really be able to track the location of anyone in the country, all the time, without even obtaining a warrant?

Nate Freed Wessler, a lawyer at the ACLU, argued the case before the Court. In an interview with The Daily Beast shortly before the decision came down, he told me that the case is important for two reasons.

“Most immediately, this case addresses the protections we have in our voluminous quantities of location info held by our cellphone companies. Those records can show where we go over the course of every day for years on end, reveling extra details about activities relationships and patterns of life.”

“More broadly,” Wessler continued, “this is the Supreme Court’s first opportunity in four decades to address the fundamental question of how the Fourth Amendment protects highly sensitive personal records held by the companies that we have relationships with.”

That question is of interest not only to privacy nerds but is now squarely in the mainstream, particularly in the wake of the Facebook-Cambridge Analytica data scandal. That issue isn’t affected by Carpenter, because it was about two private companies doing business, rather than the government. But it highlights the ways in which, to live in 21st century world, we all share more and more information without even knowing it.

“We can’t go about our lives without leaving these highly revealing crumbs behind us,” Wessler said. “It’s much more than location data: it’s the content of emails, information about homes collected by smart devices (for example, smart thermostat that knows which rooms you’re in), even the state of our bodies—heartrate data from a smartwatch which we share with Apple, for example.”

The decision in Carpenter “allows us to live our modern lives with an assurance that police will have to go to a judge to get a warrant before dipping into this really sensitive information.”

There were good reasons to doubt that the Court would rule this way, however.

First, today’s decision is a clear break from the court’s precedents on information shared with third parties, set forth in the Smith and Miller cases. But, wrote the Chief Justice, “when Smith was decided in 1979, few could have imagined a society in which a phone goes wherever its owner goes, conveying to the wireless carrier not just dialed digits, but a detailed and comprehensive record of the person’s movements.”

Indeed, Chief Justice Roberts continued:

The Government’s position fails to contend with the seismic shifts in digital technology that made possible the tracking of not only Carpenter’s location but also everyone else’s, not for a short period but for years and years. Sprint Corporation [the cell carrier in this case] and its competitors are not your typical witnesses. Unlike the nosy neighbor who keeps an eye on comings and goings, they are ever alert, and their memory is nearly infallible. There is a world of difference between the limited types of personal information addressed in Smith and Miller and the exhaustive chronicle of location information casually collected by wireless carriers today.

Second, the court’s conservative wing—apparently not including Chief Justice Roberts—has grown increasingly skeptical of the way privacy rights are understood today. As I noted back in November, when Carpenter was argued, the court’s conservative wing is increasingly enamored of “originalism,” a faux-historical doctrine that decides cases based on what the Founders supposedly intended back in 1791.

That doctrine is actually highly selective—the Founders would have banned nearly all corporations, for example—but it has become dogma in the far-right circles of the Federalist Society, the source for Donald Trump’s far-right judicial nominees, including Justice Gorsuch, because it would eliminate 20th and 21st century applications of constitutional principles to newly articulated issues such as abortion, LGBT equality, and racial discrimination.

Needless to say, the Founders didn’t say much about cellphone location data. Indeed, the text of the Fourth Amendment only protects “[t]he right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures.” Is cellphone location data a “paper” or “effect?” Maybe, but that hardly seems like “originalism” anymore.

For these reasons, Justices Thomas, Alito, and Gorsuch each (separately) dissented from the majority opinion. (Justice Kennedy also dissented, but on different grounds: that the location data is imprecise enough that it is not really private.) With abundant citations to 18th century texts, each of them held that because the location records are the property of the cellphone company, not the individual user, people like Carpenter have no privacy rights in what they contain. You have privacy rights in your property, period.

Interestingly, Justice Gorsuch speculated that it might be possible to articulate a property interest in records like cellphone location data, even if it’s not exactly your property. Unfortunately, since no one but Justice Gorsuch thinks this way, that theory was not put forward in this case.

Chief Justice Roberts, however, was the real surprise. Unlike the three most conservative justices, he declined to adopt a restrictive, “originalist” theory of Fourth Amendment Rights. But unlike Justice Kennedy, he eviscerated the doctrine that if you share information with others, you no longer have a privacy interest in it. That doctrine may have made sense in 1979, but it doesn’t make sense today.

In some ways, though, the Court’s opinion is more conservative than the conservative dissents. There are many kinds of conservatives after all: law-and-order conservatives supported the government in this case, but libertarian conservatives opposed it. And the Court’s opinion is a civil libertarian rallying cry. For digital rights, is as much a clarion call as the Obergefell case was for LGBT rights.

It is also judicially conservative, which has been a hallmark of the Roberts court.

“Our decision today is a narrow one,” the Court’s opinion concluded, noting that several emerging digital privacy issues remain unresolved. But such judicial conservatism is warranted in a field as rapidly changing as this one. Concluding his case with the perfect (and not well-known) quotation, Chief Justice Roberts wrote “as Justice Frankfurter noted when considering new innovations in airplanes and radios, the Court must tread carefully in such cases, to ensure that we do not ‘embarrass the future.’”