Is a mobile home more like an automobile or a house? This was the key question that the justices of the US Supreme Court had to determine in California v. Carney, a 1985 case about the warrantless search of a mobile home parked in a lot in downtown San Diego. An ordinary person’s answer to the question would be “both.” But the law often relies on mutually exclusive categories. Whether the mobile home was an automobile or a house mattered because under the Fourth Amendment, law enforcement needed a warrant to search a house but not to search a car. If the justices were to decide that the mobile home was a house, then—because the DEA agents in the case conducted a warrantless search—the marijuana that they found could not be used as evidence against the defendant Carney. Without evidence, the prosecution no longer had a case.

Oral argument in Carney began abstractly, almost philosophically. What makes a car a car? The government maintained that a car’s essence was its mobility, the capability for “self-locomotion.” The defense argued that even with wheels, the more that a car takes on the attributes of a house—like having window drapes, upholstered furniture, a kitchen—then it becomes one. After the lawyers staked their positions, the justices peppered them with hypothetical questions that tested the limits of their competing definitions. What if the mobile home sat in a trailer park and was hooked up to water and electricity? What if a mobile home, even one with drapes and furniture, was traveling on a public highway?

Ultimately, a majority on the court decided that a mobile home, so long as it had wheels, was a car for Fourth Amendment purposes. But the opinion rested its conclusion not just on “ready mobility.” The more fundamental reason was that motorists had “reduced expectations of privacy” because of “the pervasive regulation of vehicles capable of traveling on the public highways.” In other words, what made a car a car was government regulation.