“Knock, knock. It’s the police; can we come in?” Word to the wise: If you say no, they can’t.

But what if they put you in handcuffs and drag you away? Can they then come back and get permission from your live-in partner? You may think the answer should be an obvious “No!” - - but it isn’t. The appeals courts are all over the map on the question, and the U.S. Supreme Court, in Fernandez v. California, has just heard arguments over whether your undoubted right to keep the police out of your home ends when they haul you away.

On the surface, it seems preposterous that the police could get around your privacy rights by arresting you. After all, the Fourth Amendment was designed precisely to protect people whom the authorities wanted to take into custody. But law enforcement is nothing if not creative when it comes to finding ways around the Constitution -- and domestic violence makes the whole issue far more complicated than it would otherwise be.

The legal background is actually pretty interesting. In 1974, in U.S. v. Matlock, the court held that a spouse, roommate, or live-in boyfriend or girlfriend could give the police permission to enter a private dwelling. The theory was that any resident could waive the privacy of the others and that it was unreasonable to make the police find the primary owner to get permission.

Respecting Privacy

But in the 2006 case of Georgia v. Randolph, the court clarified that if both residents were present and one invited the police in while the other ordered them to stay out, the police could not enter. The opinion was written by Justice David Souter, who combined strong respect for privacy, exquisite manners and lifelong bachelorhood (I know because I worked for him, though not at the time of the case). To Souter, it seemed obvious that “no recognized authority in law or social practice” gave one resident priority over the other. The privacy of the home was paramount, and someone standing outside facing a disagreement between co-tenants would not feel invited in. Under these circumstances, those inside enjoyed a reasonable expectation of privacy.

This result did not satisfy police or advocates for domestic-violence victims, who believe that the typical co-tenant disagreement is when an abusive husband tells the cops to stay out while the battered wife invites them in. The solution was obvious: Arrest the husband and remove him from the scene. That would avoid the problem of the Randolph holding. Then, under the authority of the Matlock case, the wife could invite the police in to look for evidence.

This kind of legal chicanery in a good cause is a common problem for the constitutional law of criminal procedure. In a case called Missouri v. Siebert, the court struck down the so-called “Missouri two-step,” in which police try to get around the Miranda warning by questioning a suspect without it, then Mirandizing him and asking him the same questions again. Both the George W. Bush and Barack Obama administrations have done something similar with terrorism detainees, the first subjecting them to Central Intelligence Agency interrogation techniques (often for months or years), then giving them Miranda warnings and trying to introduce their testimony in court.

When it comes to removing the husband, the intent is similar: to bring about a just result by avoiding a pesky constitutional protection.

Battered Wife

In the Randolph case, Souter insisted that domestic-violence concerns should not come into play, because the decision to arrest a husband who is battering the wife is different from the separate question of gathering evidence after the fact. This was formally correct but not perhaps entirely realistic, because conviction might depend at least in part on evidence gathered inside the home where the battery took place. In the case now in front of the Supreme Court, the evidence gathered after the husband was arrested had nothing to do with the domestic dispute that got Walter Fernandez arrested -- it was evidence of drugs that the police wanted and Fernandez’s wife was happy to let them see once he was off the scene.

It’s hard to feel too sympathetic to Fernandez, who could’ve avoided the search by not hitting his wife and giving the cops the chance to arrest him. But two-step constitutional games are a bad idea, even when motivated by the laudable desires to protect women from domestic abuse and send terrorists to prison.

The whole point of the procedural protections imposed by the Constitution is that they cover not only the innocent, but also the guilty. The right to privacy in one’s own home, one of the most basic liberties enshrined in the Constitution, should exist when I insist on it. Arresting me to get me out of the picture is a game. And fundamental rights are not to be toyed with.

(Noah Feldman, a law professor at Harvard University and the author of “Cool War: The Future of Global Competition,” is a Bloomberg View columnist. Follow him on Twitter at @NoahRFeldman.)

To contact the writer of this article: Noah Feldman at noah_feldman@harvard.edu.

To contact the editor responsible for this article: Tobin Harshaw at tharshaw@bloomberg.net.