On March 16, 1976, police officers in Baltimore, Maryland, spotted a man driving a Chevrolet Monte Carlo; the car matched one a witness to a crime had seen drive slowly by her house. By noting the license plates, they were able to get the home phone number of the driver, Michael Smith. On August 22, 2009, in a separate case a continent away, police officers stopped a Lexus making its way through the Lincoln Park neighborhood of San Diego, California, with expired tags; the license of the driver, David Riley, wasn’t valid, either, and there were two guns under the hood of the car. The police also took his Samsung Instinct M800 smartphone which, they said, had been in his pocket.

There is a single trajectory that joins the car ride in Baltimore and the one in San Diego, thirty-three years apart, by way of two major Supreme Court cases: Smith v. Maryland, decided in 1979, which said that the police were within their rights to trace Smith’s phone calls without a warrant (and is often cited in defense of the National Security Agency’s bulk collection of data on Americans); and Riley v. California, decided last week, in which the Court said that the police were not right to trawl through the data on Riley’s cell phone—texts, contact lists, pictures, videos of street-boxing bouts—without a warrant. The decision in Riley was unanimous and essential. The opinion, written by Chief Justice John Roberts, did not mention the N.S.A., but it reflects the debate about the agency that has taken place in the past year, thanks to documents leaked by Edward Snowden, an N.S.A. contractor.

So how might Riley v. California affect the cases that are surely headed to the Supreme Court, post Snowden? Jeffrey Toobin and I talked about this question, along with others related to the Court’s recent decisions, in this week’s Political Scene podcast. (I should say that Toobin and I have a somewhat different view on the subject, in part because of the gulf he sees between national-security and criminal-justice jurisprudence.) These cases, together, will help define the future of the Fourth Amendment, which affirms “the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures” in the absence of a warrant. They also touch on questions of language and technology, and the way one shapes the other.

Here are four ways Riley matters when thinking about the N.S.A.:

1. A phone is not a phone. Or, rather, it is only accidentally called one. “The term ‘cell phone’ is itself misleading shorthand,” Roberts wrote in his opinion. “Many of these devices are in fact minicomputers that also happen to have the capacity to be used as a telephone. They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers.” Roberts’s language here is very much like that of Judge Richard Leon, of the D.C. District Court, who noted, in an opinion last year, “Cell phones have also morphed into multi-purpose devices. They are now maps and music players.… They are cameras.… They are even lighters that people hold up at rock concerts.” One of the N.S.A.’s sleights of hand, for which it had the stage help of the Foreign Intelligence Surveillance Court, was acting as though a police man standing next to a telephone technician in 1976 to find out what number a single, identifiable suspect was dialing on a single rotary phone was legally indistinguishable from the mass collection of records of the cell-phone calls of virtually all Americans—phones, records, nothing else to see. For Leon, the non-phoneness of modern phones rendered this absurd. Roberts’s opinion suggests that this shelving of Smith v. Maryland might find some sympathy on the Court.

2. A phone is also not a cigarette pack filled with heroin. In United States v. Robinson, a 1973 decision that also involved the police pulling over a car—a 1965 Cadillac, in this case—the court found that a police officer could search a soft cigarette pack he came across patting down a man named Willie Robinson, Jr. The pack, which the officer opened because it felt strange, turned out to contain fourteen hard capsules of heroin. That falls in the category of searches that are reasonable, Justice William Rehnquist wrote in the majority opinion, to see what a person is carrying when he’s arrested, in case there’s, say, a weapon or something hazardous—and whatever was in that pack “he knew it was not cigarettes.” But when it comes to a suspect’s phone, Roberts wrote, the police know what it is and what they will find: data. Roberts explicitly rejects the idea that there are simple analogies between the search of physical objects (tangible things) and the data to which a phone is a portal. “Modern cell phones, as a category, implicate privacy concerns far beyond those implicated by the search of a cigarette pack, a wallet, or a purse.”

Similarly, Roberts points out that earlier rulings that let police search pockets were based on the assumption that a pocket couldn’t possibly hold very much. But, he writes,

If his pockets contain a cell phone, however, that is no longer true. Indeed, a cell phone search would typically expose to the government far more than the most exhaustive search of a house: A phone not only contains in digital form many sensitive records previously found in the home; it also contains a broad array of private information never found in a home in any form-unless the phone is.

New technology doesn’t mean that law enforcement gets a bonanza; it means that precedents need to be reëxamined before they are uncritically applied—and that the N.S.A.’s advocates will have some real work to do. As Roberts puts it, “Any extension of that reasoning to digital data has to rest on its own bottom.”

3. If it’s “My House,” it is not “just” metadata. Since the Snowden revelations, we have been constantly reassured that much (though not all) of what the government collects without a warrant is “just” metadata—and what could be the harm in that? Metadata is supposedly distinguishable from content—it is information about a communication, the labels affixed to it, the addresses and the contacts, the times and the dates, and perhaps the locations. The reply is that an enormous amount can be learned from putting pieces of metadata together, and an awareness of that is present in Riley v. California and its companion case, United States v. Wurie. After the police took Brima Wurie’s phone, they saw calls from a number that came up as “My House”; that was used to connect him to evidence found at the address associated with the number. Police looking at Riley’s phone saw that some names on his contact list were designated “CK,” which they took to be shorthand for “Crip killer,” and part of their evidence for his affiliation with the Bloods gang. (Another piece of evidence was a photo on his cell phone of Riley with yet another car, a red Oldsmobile—the Blood’s color—that had been at the scene of a gang shooting.) “We also reject the United States’ final suggestion that officers should always be able to search a phone’s call log, as they did in Wurie’s case,” Roberts wrote, noting that “the Government relies on Smith v. Maryland”: