But since a different federal judge has just invoked Smith while reaching a contrary conclusion—that the Fourth Amendment does not prohibit bulk metadata collection—it's worth dwelling on what embracing the NSA's reading of Smith would really mean, and why doing so is an unsound approach to interpreting the Constitution, even if we forget about the present controversy and speak more generally.

To illustrate what I mean, indulge a hypothetical I dreamt up while trying to articulate to myself why I find the way Smith is being invoked so illegitimate and dangerous.

Like most Americans, I go with some regularity to the barber shop. I sit down in the chair, the barber goes to work with his scissors, and by the end of the cut my hair is strewn about the floor. As I pay at the register, the barber sweeps up my hair and dumps it into a trashcan. It subsequently goes out to the dumpster in back. By the logic of Smith, I have no reasonable expectation of privacy as regards this hair. I freely allow it to be clipped from my head and to fall to the floor, knowing full well that third parties will take possession of it en route to the dump.

For the sake of this hypothetical, let us imagine two technological advances that take place 10 years apart. In 2015, a shampoo is introduced that prevents any hair washed with it from revealing the DNA of the person to whom it belongs. Widely used in barber shops, it renders the fact that I have no reasonable expectation of privacy in the hair I leave on the barbershop floor completely unproblematic. There is no scenario in which authorities could perpetrate serious, widespread abuses using hair taken from the floors of barbershops. And anyone could easily protect the privacy of the DNA in their hair, as a 2016 Supreme Court opinion notes while upholding the police's ability to gather hair from dumpsters.

But in 2025, another unexpected innovation occurs. Using technology unimagined in any prior year, analysts can determine, given a strand of human hair, every memory possessed by the owner of the hair at the moment when it was cut. And this technological advance is kept secret. Deep inside the NSA's headquarters, its employees are taking delivery from the contents of barbershop and salon dumpsters all over America, extracting the memories of millions of Americans, and creating a database where all of that intensely private data can be queried.

(All in the name of stopping terrorism, of course.)

Suddenly, the real-world consequences of the earlier precedent are dramatically different. With the ability to extract memories from hair, authorities have warrantless access to virtually everything previously protected by the Fourth Amendment: contents of phone calls, visuals of what goes on inside people's bedrooms, etc. The guarantee of being secure against unreasonable searches is all but meaningless now that police can examine the full contents of virtually everyone's memory in a process not considered a search for Fourth Amendment purposes.