Nojeim argued that “If strict application of the doctrine ever served us well, it no longer does, leading to absurd results. This is particularly true in an age where so much more information is communicated through intermediaries.” Kerr countered by stating that “I think that the much-maligned third-party doctrine is a critical tool for applying the Fourth Amendment to new technologies in some cases, but that it should not be extended to all cases ... Importantly, my defense of the third-party doctrine implies an important limit: The doctrine should apply when the third party is a recipient of information, but it should not apply when the third party is merely a conduit for information intended for someone else.”

Before June 2013, discussions about the third-party doctrine typically involved information relating to small numbers of people, all of whom were suspects. The thought experiment used by Nojeim and Kerr to frame their debate was: “If a suspected thief has left written records of his crime in a friend’s desk, can the police simply subpoena the friend for the records in the desk or should that be treated as a search of the suspect’s property?”

That changed with the June 2013 publication of information about the NSA’s bulk phone “metadata” collection program involving millions of phone customers, only an infinitesimal fraction of whom could reasonably be considered suspects. Metadata includes information such as the telephone numbers at both ends of a call as well as call start time and duration, but does not include the audio contents. Unsurprisingly, the news stories about the NSA metadata program led almost immediately to lawsuits challenging its constitutionality.

Faced with the task of determining whether the program is constitutional, lower court judges bound by Supreme Court precedent have a choice. They can conclude that the metadata collection is analogous—though obviously on a completely different scale—to the pen register in the 1979 Smith case, and find it to be constitutional in accordance with the third party doctrine. This is what Judge William H. Pauley III of the Southern District of New York did in his December 27 ruling dismissing the ACLU’s lawsuit in ACLU et al. v Clapper et al.

Alternatively, judges can conclude that the NSA program is probably not constitutional, but in doing so they need to explain why they believe the program falls outside the scope of the doctrine. This is what Judge Richard J. Leon of the U.S. District Court for the District of Columbia did in his December 16 ruling [PDF] granting (and immediately staying pending appeal) an injunction against the NSA in Klayman et al. v. Obama et al. What a lower court judge cannot do, however, is decide that the third party doctrine should simply no longer apply.

Thus, the courts that will hear the initial appeals of both of the above rulings are in an unenviable and all-too-common position: They will need to apply legal precedents that predate the digital era to questions that are arising in large part precisely due to the stunning advances in digital technologies in recent years. It’s a challenging exercise.