In both cases, the government argued that the searches were permissible under a long-established exception to the Fourth Amendment, which generally requires the police to get a warrant before searching “persons, houses, papers, and effects.” After a lawful arrest, however, the police may search a person’s body and immediate surroundings without a warrant, both for their own protection and to prevent the destruction of evidence.

Cellphones have upset that balance, as the court rightly recognized. First, nearly everyone has one and uses it daily. They have become so prevalent so fast that, as Chief Justice Roberts wrote, “the proverbial visitor from Mars might conclude they were an important feature of human anatomy.” More important, they contain “vast quantities of personal information,” from financial and medical records to archives over many years of private correspondence and records of places the owner has been. They are “minicomputers,” the court said, that contain more information than entire houses once did.

In short, the expectation of privacy in a phone’s contents outweighs the immediate needs of law enforcement.

Particularly since police officers may still conduct a warrantless search of a phone in a true emergency, the court was equally unconvinced by the government’s other arguments — that a phone’s incriminating data could be remotely wiped, for instance, or that it could alert an officer to approaching accomplices who might threaten his safety. These scenarios were hypothetical, the court said, or they could be addressed by existing technology.

The court acknowledged that cellphones are widely used by criminals, and that its ruling would have an impact on law enforcement’s ability to fight crime. But it said the warrant requirement is “not merely an inconvenience to be somehow weighed against the claims of police efficiency.”