Tuesday's US supreme court arguments involved a seemingly basic legal question about the future of the Fourth Amendment: do police officers need a warrant to search the cellphone of a person they arrest? But the two privacy cases pit against each other two very different conceptions of what it means to be a supreme court in the first place – and what it means to do constitutional law in the 21st century.

"With computers, it's a new world," several justices reportedly said in the chamber. Are they ready to be the kinds of justices who make sense of it?

Cellphones expose so much of our most personal data that the decision should be a 9-0 no-brainer. The basic problem that makes it a harder call is that lawyers and judges are by training and habit incrementalists, while information and communications technology moves too fast for incrementalism to keep up.

Judges wear legal professionalism and precedent as a mantel that secures legitimacy for their decisions. It's how they distinguish themselves from politicians or administrative agencies, while wielding power that is sometimes much greater than those democratically accountable actors. But this kind of narrow legalism simply cannot do when the world is changing as rapidly as it is today: all narrow analogies will systematically fail to preserve the values they did five or ten years ago, especially when we're walking around with all the metadata coming out of the bank/medical monitor/full-on GPS trackers in our pockets.

One of Tuesday's cases, United States v Wurie, stems from an arrest in 2007, the same year the first iPhone was launched and changed handheld computing forever. In the second case, Riley v California, the arrest and search happened only two years later. So the future of the Fourth Amendment will therefore depend not on the particular doctrines of the case, but on whether the formalistic justices – the ones with a line of reasoning that lacks agility to deal with a radically changing world – prevail over those who take a broader constitutional vision. If they do, we are in for a dramatic erosion of constitutional privacy protections in the coming years – all thanks to the same kind of old-school legal approach that allowed National Security Agency lawyers to justify mass telephony meta-data surveillance.

Should spy-level sway continue to hold when it comes to getting pulled over on the side of the highway?