Fourth Amendment jurisprudence is frequently accused of doctrinal incoherence. A primary reason is the persistence of two competing conceptions of "unreasonable" search and seizure. The first is libertarian in character; it understands the Fourth Amendment's command of reasonableness as establishing a constitutional boundary on investigative powers. On this view, the prohibition on unreasonable search and seizure keeps society free by limiting the government’s investigative reach. The second conception understands the Fourth Amendment's prohibition as freedom against unjustified government intrusion. This conception of reasonableness is essentially pragmatic in character, balancing liberty and law-enforcement interests.
This article interrogates these competing conceptions by focusing whether a "binary search" should be regarded as unreasonable under the Fourth Amendment. Binary search techniques reveal no more than whether there is probable cause to believe that an otherwise concealed area contains contraband or other evidence of criminality. In a binary search, the competing conceptions of the Fourth Amendment point toward different outcomes. On the libertarian conception, the Fourth Amendment would regard as unreasonable an effectively unlimited power to scrutinize otherwise private space through binary techniques. A libertarian Fourth Amendment demands limitations on binary searches. On the pragmatic conception, a binary search that discloses nothing more than the probable presence of contraband is supported by powerful law-enforcement interests, and is unlikely to threaten any legitimate liberty interest of the innocent. Therefore, it could readily be regarded as constitutionally reasonable even if unsupported by individualized suspicion of wrongdoing. As technology advances, the binary search will become increasingly important, as increasingly sophisticated and focused technologies are developed that can precisely target the presence of contraband or other evidence of criminality in both real and cyberspace.
In last Term's decision in Florida v. Jardines, the Court considered a type of binary search – the use of a trained narcotics-detection dog. In Jardines, a bare majority embraced the libertarian conception. Writing that “[a]t the Amendment’s very core stands the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion,” the Court held that “[t]he government’s use of trained police dogs to investigate the home and its immediate surroundings is a search within the meaning of the Fourth Amendment.” Jardines’s libertarianism, however, is stunted and incomplete. The Court’s holding can be readily circumvented by investigative techniques that stop short of a physical intrusion, and might even leave binary searches unrestricted if they are ultimately deemed constitutionally reasonable, even if considered a “search.” Indeed, given our current understanding of the scope of governmental regulatory power, the binary search demonstrates the absence of a coherent Fourth Amendment libertarianism. Searches with no potential to compromise the interests of the innocent – such as a binary search – are constitutionally unobjectionable in a regime that recognizes no legitimate interest in even the entirely “private” possession of items that the government may deem unlawful to possess. Thus, the persistence of the libertarian conception comes with the inevitable cost of a loss of conceptual coherence. Moving beyond the particulars of the binary-search debate, this article concludes that although the pragmatic conception leaves plenty of room for debate over the proper scope of investigative authority, only the pragmatic conception has any real power to rationalize Fourth Amendment jurisprudence, especially as it faces unparalleled challenges as a consequence of technological advance.