One of the most surprising decisions of the Supreme Court term just concluded was Justice Antonin Scalia's ruling in favor of a criminal defendant who claimed that a thermal imaging device violated his Fourth Amendment rights. The police used the device to measure the heat leaking from Danny Kyllo's house and inferred from that information that he was growing marijuana inside with heat lamps. Indeed, he was, as the subsequent search revealed: more than 100 plants' worth.

In the most unlikely collaboration of the year, Justices Scalia and Clarence Thomas joined forces with Justices Ruth Bader Ginsburg, Stephen Breyer and David Souter to rule that use of the thermal imaging device was an unconstitutional search. The decision is surprising in several respects--and not just because it rules for a drug defendant. It announces a bright-line rule barring the use of high-tech devices to intrude upon the privacy of the home, in an era where the Court has largely abandoned bright-line rules except where they benefit the police. It speaks in majestic tones about protecting privacy from the onslaught of technology, from a Court that has all but given up on privacy in favor of crime control. And it reaches a result that was by no means foreordained. This was a close case, as Justice John Paul Stevens's quite reasonable dissent shows.

So what's going on here? Should liberals (or drug manufacturers) start looking to Justices Scalia and Thomas for protection of criminal defendants' rights? I'm afraid not. This is a rare instance of an alliance between liberals and libertarians, united here in support of the sanctity of the home. For Scalia and Thomas, at least, it all comes down to property. Step outside, and you're fair game.

The dispute centered on whether the use of the thermal imaging device was a "search" that invaded Kyllo's "reasonable expectation of privacy." The police argued that the device merely registered information from the outside of the home. A police officer's observation that snow melted more quickly on certain parts of a roof would provide the same information, but no one would call that a "search." Since the information came from outside the house, it invaded no privacy.

Justice Scalia rejected that approach, and concluded that whenever the police use "sense-enhancing technology" not in general public use to obtain information that they otherwise could not have gathered without entering the home, they have conducted a search, for which they must have probable cause and a warrant. His opinion waxes eloquent on the home as castle and the need to protect citizens from the intrusions of modern technology. (None too soon, as police are already working on ultrasound technology for houses, although one wonders how they're going to apply petroleum jelly to aluminum siding.)

In its attempt to protect privacy from advancing technology, the decision is a landmark and will stand along with the Warren Court's 1967 decision in Katz v. United States, which extended the Fourth Amendment to include wiretapping. But in another respect, the decision marks an ironic return to the pre-Katz world. Before Katz, Fourth Amendment law was governed by property notions, leading the Court to make ridiculous distinctions between listening devices attached to an outside wall with a thumbtack, which were said to invade property and require a warrant, and similar devices merely taped to the wall, which were deemed not to invade property and therefore not to require a warrant.

Katz importantly held that the Fourth Amendment protects "people, not places" and eschewed arcane property questions for an inquiry into whether the government had invaded a person's "privacy." But Kyllo brings us back full circle, because without any reasoned explanation it expressly limits its protection to homes. Justices Scalia and Thomas's libertarian instincts stop at the doorstep. A man's home may be his castle, but in the view of these Justices, at least, the streets still belong to the police.

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