Strike a blow for privacy.  In a rare unanimous decision in an important Fourth Amendment case, the Supreme Court ruled on Wednesday that police officers who arrest someone must get a warrant before searching their cellphone.

At some level, Riley v. California revives the importance of warrants: decisions by a neutral judge about who and what may be searched.  The Fourth Amendment generally requires warrants based on “probable cause” before the government may search a person or property.  But the warrant requirement has more holes than Swiss cheese.  Over the years, courts have created any number of exceptions: searches incident to arrest, searches under exigent circumstances, searches of items in plain view, searches of glove compartments in cars, among many others.  When police officers effect an arrest, they usually need no warrant to search the person for weapons or evidence of a crime: a gun, a razor blade, a bag of drugs.

Then why do cellphone searches require warrants?  The Court’s short answer: cellphones are not themselves dangerous and rarely can the arrestee delete incriminating evidence from the phone while in handcuffs.  The long answer is more interesting, and reflects the Court’s growing sensitivity to digital privacy—its recognition that in the digital age, government must be restrained.  Different rules apply because digital information is sweeping, revealing, and personal on a scale never before contemplated.  As the Court held, “[m]odern cell phones, as a category, implicate privacy concerns far beyond those implicated by the search of a cigarette pack, a wallet, or a purse.”

For one, “a cell phone collects in one place many distinct types of information—an address, a note, a prescription, a bank statement, a video that reveal much more in combination than any isolated record.”  For two, “a cell phone’s capacity allows even just one type of information to convey far more than previously possible.  The sum of an individual’s private life can be reconstructed through a thousand photographs labeled with dates, locations, and descriptions; the same cannot be said of a photograph or two of loved ones tucked into a wallet.”  This data may date back years, even decades.  And finally, cellphones have data “qualitatively different” from physical items: “Internet search and browsing history” revealing an individual’s “private interests or concerns,” and “[h]istoric location information” that can “reconstruct someone’s specific movements down to the minute.”

Could anyone seriously argue that, simply by arresting someone for an expired car registration, an officer has the automatic right to access her entire life: emails, browsing history, travel history, calendar, photos, and more?  Yet that was the argument of the Obama Administration, a position the Court thankfully and resoundingly rejected.

Riley ends with an appeal to histor­y.  “The Fourth Amendment was the founding generation’s response to the reviled ‘general warrants’ and ‘writs of assistance’ of the colonial era, which allowed British officers to rummage through homes in an unrestrained search for evidence of criminal activity.”  According to the young John Adams, when James Otis gave a speech in 1761 denouncing writs of assistance, “[t]hen and there the child Independence was born.”

Will Riley mark a true awakening of digital privacy?  As noted in “Surveillance USA,” the government has admitted to monitoring and collecting the telephone metadata of tens of millions of ordinary Americans: who calls, who answers, when, how often.  The National Security Agency is also collecting the content of millions of Internet communications of ordinary Americans.  These are Constitutional violations of breathtaking scope.

The government is hardly alone.  Dozens of companies—some well-known, many virtually unknown—collect vast amounts of information about our Internet habits: where we search, who our friends are, what we do, what we email, what we think.  Statutory protections against this unprecedented invasion of our personal lives are weak or non-existent.

“Knowledge is power,” Francis Bacon famously observed over four centuries ago.  In the digital age, information is power, but we don’t control our own information, or in most cases, even know who does.  So this week, let us give thanks to the Supreme Court.  They have given Americans just a little power back.