Can the government access records of your cellphone’s location history over a period of weeks, months, even years, without obtaining a warrant? On Friday, the Supreme Court ruled forcefully that it cannot in Carpenter v. United States, perhaps the most important Fourth Amendment decision of the 21st century so far. Chief Justice John Roberts’ majority opinion—joined only by the court’s liberals in a 5–4 ruling—has been widely praised for putting real constitutional limits on the modern surveillance state. But Roberts’ decision drew heavily from ideas put forth by Justice Sonia Sotomayor in a prescient concurring opinion six years ago. Indeed, Carpenter might not have been possible had Sotomayor not already laid the groundwork for a dramatic expansion of digital privacy.

Carpenter’s key predecessor is United States v. Jones, a 2012 decision that involved a similar clash between contemporary technology and ancient constitutional principles. In Jones, a police officer attached a GPS tracking device to the bottom of a Antoine Jones’ car without a valid warrant, then tracked its movement for 28 days. Prosecutors used this GPS data to connect Jones to a stash house; a jury convicted him of drug trafficking, and he was sentenced to life in prison. Jones argued that the government’s warrantless collection of GPS data violated his Fourth Amendment right against unreasonable searches and seizures.

The Supreme Court agreed—but splintered in its reasoning. Justice Antonin Scalia’s majority opinion held that law enforcement had committed a search when it attached the GPS device to the underside of Jones’ vehicle. “When the government physically invades personal property to gather information,” Scalia held, “a search occurs,” and a warrant is typically required. An unusual coalition of justices signed onto his opinion: Chief Justice John Roberts, as well as Justices Anthony Kennedy, Clarence Thomas—and Sonia Sotomayor.

In an acerbic opinion concurring in the judgment, Justice Samuel Alito admonished Scalia for fixating on the cops’ “trespass” upon Jones’ car. Scalia, Alito wrote, “disregards what is really important”—the “long-term tracking” of Jones—and “instead attaches great significance” to something “trivial”: attaching a “small, light object” to a car. Instead of focusing on law enforcement’s physical trespass, Alito asserted, the court should’ve grappled with the “long-term monitoring” of Jones’ movements. It was this surveillance, he wrote, that infringed upon Jones’ Fourth Amendment rights, not the initial placement of the GPS tracker.

Although Sotomayor joined Scalia’s opinion, she wrote separately to bridge the gap between Scalia and Alito—and, in the process, plot out a path forward for future defendants. Sotomayor, who is fiercely defensive of property rights and personal privacy, agreed that the police committed a search by sticking a GPS tracker on Jones’ car. But, she noted, “physical intrusion is now unnecessary to many forms of surveillance.” Increasingly, the government may surreptitiously “assemble data that reveal private aspects of identity” without any “physical invasion on property.”

Sotomayor’s opinion in Jones, which runs a mere six pages, proved hugely influential.

It would seem, the justice continued, that Americans should have a reasonable expectation of privacy “in the sum of one’s public movements,” compelling the government to get a warrant before tracking someone’s location. After all, by merely examining GPS data, the government can learn all about our “familial, political, professional, religious, and sexual associations”: our trips to an abortion clinic, a mosque, a gay bar, and so on. But there’s a problem: The Supreme Court has held that individuals have no expectation of privacy in information they turn over to third parties, such as business records and call logs. “It may be necessary,” Sotomayor concluded, “to reconsider” this “third party doctrine” in the digital age. Many Americans turn over troves of data to internet providers and phone companies; most would still assume they have a right to privacy in that information. A rule crafted to fit 1970s technology just doesn’t make sense in the 2010s.

Sotomayor’s opinion in Jones, which runs a mere six pages, proved hugely influential. Hundreds of lower courts have cited it in decisions involving the clash of technology and privacy. In 2014, Roberts cited it in a landmark opinion holding that the police may not search cellphones without a warrant. And when Timothy Carpenter took his case to the Supreme Court, Sotomayor’s Jones concurrence quickly emerged as a central battleground. The ACLU, which represented Carpenter, quoted from it extensively, as did several amicus briefs filed on Carpenter’s behalf. At oral argument, Sotomayor dominated the debate.

It’s easy to see why. Carpenter revolves around cell site location information—records of every place your cellphone has been, which wireless carriers store for years. A federal law allows the government to access this data without a warrant. Yet the information reveals a great deal about cellphone users, allowing law enforcement to learn the details of their private lives without committing a physical search. In other words, Carpenter involves the exact kind of information that Sotomayor fretted about in Jones: deeply revealing data that the government can access without touching your property or getting a warrant, just because a third party possesses it.

And on Friday, a majority of the court heeded Sotomayor’s warning, extending Fourth Amendment protections to cell-site location information. Roberts assigned the opinion to himself—that’s his prerogative as chief justice—but cited his colleague repeatedly and drew from her reasoning in Jones. Individuals “have a reasonable expectation of privacy in the whole of their physical movements,” Roberts wrote, channeling his inner Sotomayor. “Allowing government access to cell-site records contravenes that expectation,” giving the government “an all-encompassing record of the holder’s whereabouts.” Every smartphone user has “effectively been tailed every moment of every day” by this “tireless and absolute surveillance.” And under the Fourth Amendment, the government may not access these records without a warrant supported by probable cause of criminal activity.

Oddly enough, Alito dissented from Roberts’ opinion, apparently unwilling to follow his own Jones concurrence to its logical conclusion. But that matters little: Sotomayor seems to have persuaded Roberts, and the chief justice, in turn, sharply limited third-party doctrine for the digital era. His Carpenter opinion has Sotomayor’s fingerprints all over it—as does the judiciary’s broader jurisprudence. Six years ago, in just six pages, Sotomayor laid out a potent theory of the Fourth Amendment. On Friday, her prophetic vision of modern privacy became the law of the land.