I attended the Supreme Court’s oral argument in U.S. v. Jones today, the case dealing with the Fourth Amendment constitutionality of using GPS to track individuals’ movements without a warrant. Predicting outcomes is fraught, and you’re getting your money’s worth from the following free observations.

It seemed to me that most members of the Court want to rule that the government does not have free reign to attach GPS devices to cars. Justices Kennedy, Breyer, and Sotomayor, for example, noted the vast consequences if the government were to win the case. Law enforcement could attach tracking devices to people’s overcoats, for example, and monitor their movements throughout society without implicating the Fourth Amendment. Voluble as he often is, Justice Scalia did not say that the Fourth Amendment doesn’t reach GPS because GPS data wasn’t around for the Framers to insulate from government access.

Justice Alito’s thinking seemed to venture the furthest. He noted how insufficient it would be if the Court were to decide the case based on the narrow ground that attaching a GPS device to a car is an unreasonable seizure. Doing so would not account for the vast amount of personal data the government might access without attaching something to a car, clothing, or other property. If not in this case, the Court will soon have to face the (pernicious) third-party doctrine, which holds that a person has no Fourth Amendment interests in information shared with others.

If the Court desires to rule against the government, the one thing it lacks is a rationale for doing so. When it was time for Jones’s counsel to argue, the Justices seemed frustrated not to have a principle on which to base a decision.

Justice Scalia early-on declared his concern with GPS tracking and his dismay that the “reasonable expectation of privacy” test from Katz v. United States (1967) might shrink the zone of privacy the Framers sought to protect in the Fourth Amendment. But he later retreated into a sort of catch-all posture: the Congress can control GPS tracking if it wants. (Jones’s counsel cleverly suggested that there were 535 reasons not to do that.)

Other Justices’ questions danced awkwardly with the “reasonable expectation of privacy” test. Justice Kennedy was equivocal once about whether it would apply. Chief Justice Roberts seemed acutely aware of the Court’s incompetence to make judgments of such broad societal sweep. This is for good reason: there is no way to determine what society thinks, or what is “reasonable” in terms of privacy, when new technologies are applied new ways.

The solution to this conundrum can be found in the Cato Institute’s amicus brief in the Jones case. The Court should not use the “reasonable expectation of privacy” test from Justice Harlan’s Katz concurrence. Rather, it should follow the majority holding, which accorded Fourth Amendment protection to information that Katz had kept private using physical and legal arrangements. The government stands in the same shoes as the general public when it comes to private information—that is, information that can’t be accessed legally or with ordinary perception. When the government accesses information that was otherwise private, those searches and seizures must be reasonable and must almost always be based upon a warrant.

This way of administering the Fourth Amendment is not a snap of the fingers. There will be details to hash out when the Court eventually finds that having a Fourth Amendment interest in information turns on a factual question: whether someone has concealed information about him- or herself.

The biggest impediment to adoption of this rule may be getting lawyers to realize that “reasonable expectation of” is not a prefix required every time they use the word “privacy.”