For law students studying criminal procedure—or at least for those cramming for the exam—it becomes a mantra: government conduct only implicates the Fourth Amendment protection against unreasonable searches if it invades a “reasonable expectation of privacy.” This is not the contemporary definition of the word “search,” nor was it the definition at the time of the founding. But, via a well-intentioned concurrence by Justice Harlan in the famous 1967 case of Katz v. United States, it became the Court’s definition.

This lack of fealty to the English language left some questions. For example, is determining whether someone has a “reasonable expectation of privacy” a normative inquiry (what should a person expect) or an empirical one (what does a person expect)? And why do we need to determine whether someone had a reasonable expectation of privacy when the next question is, to the consternation of law students, whether the search was reasonable? Remarkably, the High Court has never answered these questions, although Justice Scalia has complained about the latter. But there are even larger problems with what the Court has done with its “reasonable expectation of privacy” test, problems that became immediately apparent, but that are now becoming critical . . . .