The Supreme Court ruled today in Florida v. Jardines that “use of trained police dogs to investigate the home and its immediate surroundings is a ‘search’ within the meaning of the Fourth Amendment.”

It’s the right result. The Court was divided 5–4, though, and the case shows some of the same fissures around Fourth Amendment doctrine that U.S. v. Jones did last year.

The majority opinion, written by Justice Scalia, won’t clear up the doctrinal debates, which are sure to continue. Instead, it retreats to the home. The specific protection for “houses” in the Fourth Amendment, he wrote:

renders this case a straightforward one. The officers were gathering information in an area belonging to Jardines and immediately surrounding his house—in the curtilage of the house, which we have held enjoys protection as part of the home itself. And they gathered that information by physically entering and occupying the area to engage in conduct not explicitly or implicitly permitted by the homeowner.

Property law gives strangers an implied license to approach a house for the variety of purposes they may have. “But introducing a trained police dog to explore the area around the home in hopes of discovering incriminating evidence is something else. There is no customary invitation to do that.”

Justice Scalia did use the case to answer a question left open by Jones. He emphasized that the “reasonable expectation of privacy” test from Katz v. United States (1967) built upon, and did not supplant, the Fourth Amendment’s foundation in property. He specifically declined to use that test in the holding.

The dissent objected vigorously to the idea that approaching the front door of a home via the walk was a trespass.

“[G]athering evidence—even damning evidence—is a lawful activity that falls within the scope of the license to approach,” Justice Alito wrote. “And when officers walk up to the front door of a house, they are permitted to see, hear, and smell whatever can be detected from a lawful vantage point.”

The dissent also rejected an argument put forward by the concurrence: that the reasonable expectation of privacy test is an alternative ground for the holding.

Yes, Justice Kagan would also have used “reasonable expectations” to decide the case, but her concurrence covers more important ground than that. As she did at oral argument, she fixed on the government’s use of the dog to perceive things that couldn’t otherwise be perceived. That’s what searching is.

“[P]olice officers came to Joelis Jardines’ door with a super‐​sensitive instrument, which they deployed to detect things inside that they could not perceive unassisted.” And later: “[A] drug‐​detection dog is a specialized device for discovering objects not in plain view (or plain smell).”

In the Cato Institute’s brief in the case, I emphasized that drug‐​dog detection was but one form of chromatography, the use of which the court should treat as searching because it “look[s] for or seek[s] out that which is otherwise concealed from view” (quoting Black’s Law Dictionary).

I also emphasized the poorly recognized twin‐​relationship in Fourth Amendment doctrine between “plain view” and constitutional searching: “A ‘search’ occurs when government agents seek out that which is otherwise concealed from view, the opposite condition from what pertains when something is in ‘plain view.’ ”

And I argued: “This Court should find a ‘search’ to have occurred in this case because the use of a drug‐​sniffing dog made perceptible to government agents what they otherwise could not perceive.”

I’m gratified to see Justice Kagan, joined by justices Ginsburg and Sotomayor, adopt this reasoning, whatever its source. The Court’s women, it seems, have the best Fourth Amendment reasoning in this case.

The result today is clearly favorable for Joelis Jardines and mildly favorable for Fourth Amendment doctrine. Any case is a good case if it declines to use the failed “reasonable expectation of privacy” doctrine. And, as was the case in Jones, the concurrence is where the real action is.