A UNANIMOUS Supreme Court declared Wednesday that, in the face of new technology that has reshaped daily life, it will not “mechanically” apply old legal doctrines that offer Americans too little protection in novel digital circumstances.

The justices ruled that law enforcement officers cannot rummage through the electronic contents of an arrestee’s cellphone. In most cases, the officers must get a warrant first. This is an unambiguously sensible decision, showing that the court will not remain inert as the country strikes anew the balance between privacy and security. Rather, it will bound the debate with serious, basic limits on electronic search and seizure.

The justices considered two cases in which police officers confiscated and examined the phones of two men they had arrested. In one case, information on a suspect’s phone led officers to search his apartment, where they found drugs, a gun and ammunition. In the other, officers inspected photos, videos and text stored on a suspect’s phone to link him to a gang shooting. Both men claimed that, since the police had not obtained warrants, taking the electronic evidence violated their Fourth Amendment protection against unreasonable search and seizure.

To press that claim, their lawyers had to overcome several Supreme Court precedents that gave law enforcement officers relatively wide latitude to search through material on or around an arrestee’s person. Officers can look at physical notes, photos and other records that happen to be in a suspect’s pocket — or anywhere within his or her reach — during an arrest.

But the court rightly refused to draw a simplistic analogy between the physical objects a person can carry and the the vast storehouse of digital material accessible through the average mobile phone. “That is like saying a ride on horseback is materially indistinguishable from a flight to the moon,” Chief Justice John G. Roberts Jr. wrote for the court. “Both are ways of getting from point A to point B, but little else justifies lumping them together.”

An Amazon representative shows off the 3D map features of the company's new Fire smartphone at the company's campus in Seattle, Washington. (Jason Redmond/Reuters)

Justice Roberts noted that, absent any restrictions on browsing through arrestees’ cellphones, officers would have access to massive amounts of information. Phone hard drives can contain galleries of pictures, thousands of pages of documents, hundreds of videos and e-mail records dating back months or more. Modern mobile phones contain all sorts of information that police never or almost never would have found on suspects when judges constructed the old rules: Internet browsing data, search histories and medical records, to name a few. The government’s old justifications for warrantless searches of arrestees were protecting police officers’ safety and securing evidence from destruction. Neither can justify unsupervised intrusion into a smartphone trove.

What’s next? The court raised concerns Wednesday about giving police free access to information Americans store on remote servers in “the cloud,” using increasingly common Web applications such as Google Docs. This suggests that the justices may have more to say about legal protections for cloud-based information, another place where analogies to old legal norms may prove inadequate.