The issue for the Court: was the dog sniff itself a search, for Fourth Amendment purposes? If so, the warrant was invalid, because there was no probable cause for it.
Justice Scalia, writing for the Court, said a sniff is a search. The front porch, he wrote, is considered by the law part of the home, an area that is called curtilage. It is true, Scalia concedes, that strangers (or for that matter police) may walk up to the door and knock. But bringing a trained sniffer dog is something else. "To find a visitor knocking on the door is routine (even if sometimes unwelcome)," he wrote. "[T]o spot that same visitor exploring the front path with a metal detector, or marching his bloodhound into the garden before saying hello and asking permission, would inspire most of us to -- well, call the police."
Scalia reaches his result much as he did the result in United States v. Jones, a 2012 case in which the Court found that placing a GPS tracker on a private car is a "search" requiring a warrant. He looks to eighteenth-century tort law and the 1765 case of Entick v. Carrington, in which English law enforcement broke into a writer's home and smashed open all his containers to search for "seditious" writings. No dogs were used in the search, but Scalia cites it for the general rule that "no man can set his foot upon his neighbor's close without his leave."
Interestingly enough, Justice Elena Kagan (in a concurring opinion joined by Justices Ruth Bader Ginsburg and Sonia Sotomayor), meets Scalia on his own rhetorical ground. In an even more conversational style, Kagan agrees with Scalia's conclusion while challenging his method. The question is not what the 18th-century Court of King's Bench would have thought of Franky, she suggests, but the test used by modern courts -- whether Jardines had "a reasonable expectation of privacy" in his front porch.
Here, Kagan has no difficulty finding such an expectation. Franky "was not your neighbor's pet, come to your porch on a leisurely stroll," she writes. He is "to the poodle down the street as high-powered binoculars are to a piece of plain glass." Because Franky is "a specialized device," the case could be decided "by looking to Jardines' privacy interests." That method would give the courts a way to fashion search rules that do not depend on ownership of land, permitting greater flexibility in evolution of the Fourth Amendment as technology progresses.
The malign march of technology is a subject that often engages Justice Samuel Alito. In Doe v. Reed, he vainly urged the Court to hold that voters who sign a referendum petition against gay marriage have a First Amendment right to keep their names secret, because the growth of the Internet would allow vindictive gay-rights advocates to find their homes and businesses. In his dissent in Jardines, however, Alito finds no similar reason for concern. The Internet may be new, but Deputy Dog is, well, just a dog. "Dogs have been domesticated for about 12,000 years; they were ubiquitous in both this country and Britain at the time of the adoption of the Fourth Amendment; and their acute sense of smell has been used in law enforcement for centuries."