Knock, knock. Civil libertarians, particularly those on the political left, were certainly rousted out of bed by the Supreme Court’s ruling on the so-called knock-and-announce requirement. Hudson v. Michigan arose from the conviction of one Booker T. Hudson on drug-related charges. The prosecution used evidence gathered during a search of his home. Hudson tried to exclude that evidence on the grounds that the search had been unconstitutional. Although the police had obtained a valid warrant, they violated the knock-and-announce rule, an age-old legal tradition that requires the officers to wait after they have knocked and announced themselves so that whoever is inside has a chance to open the door.

The question before the Supreme Court was whether the violation of that “constitutional” protection is so severe that it would justify an exclusionary rule that would keep juries from hearing evidence obtained in such a search. The court ruled that knock and announce is not so important that it would justify such a drastic step. Although the justices paid lip service to the constitutional importance of knock and announce, by taking off the table the most severe way of enforcing it, they have effectively minimized its impact.

Which is entirely within their discretion. Knock and announce is not enshrined in the text of the Fourth Amendment, where previous courts have claimed to find it. It is a common-law convention that developed in England in the Middle Ages. It was a well-established principle in England by the colonial period, but appears to have been open to interpretation for much of its history, even in modernity. So it has followed the pattern of all tenets of common law – it has gained general acceptance and judges keep it in mind, but it is open to interpretation in light of changes in society and the legal system. In some places it has been codified, as in a federal law dating from the early 20th century, and such laws will not be affected by a ruling that the procedure isn’t constitutionally mandated. Even when the Supreme Court has identified it as a constitutional protection, as it did as recently at 1995, the justices have left the door open for exceptions and on-the-fly interpretation. It has not been required in some circumstances, and the amount of time police must wait has never been firmly specified.

The last time the Supreme Court visited this issue, in its 1995 case Wilson v. Arkansas, the justices argued that knock-and-announce was on the Founders’ minds when they wrote the Fourth Amendment, and that they would have thought an unannounced search to be “unreasonable.” The Founders thought a lot of stuff they decided, in the end, against enshrining. Instead, they were quite clear about warrants, taking a full half of the amendment to lay out in detail the elements of a valid warrant. Evidently it was the warrant, and not all the other ancillary legal traditions associated with searches, they thought was the most important principle, the one thing so important they needed to specifically describe it. Knocking per se does not appear at all in the amendment.

So what the Hudson majority has done is to take a principle that has always been open to reinterpretation and reinterpret it in light of changing circumstances. They note that the increased professionalism and constitutional awareness of law enforcement, as well as the possibility of suing the government for civil rights abuses, neither of which was the case in the Middle Ages or the founding generation, now allows for less strict enforcement of the knock and announce rule, especially given that it has always only been a tradition or a simple statute instead of a constitutional provision. It turns out that while knock and announce may be policecraft, it is not constitutionally required. The left has for decades been arguing that interpretations of the text of the Constitution must be free to “evolve.” So knock, knock. Who’s there? Antonin Scalia, John Roberts, Samuel Alito, Clarence Thomas, and Anthony Kennedy.