Imagine the police arrest the driver of a car, and they handcuff and put him in the back of the squad car. The police want to search the car for evidence, but they don't have probable cause to believe there is evidence in the car. The police can't search the car based on officer safety concerns because the driver is now in the back seat of the squad car. However, yesterday's decision in Arizona v. Gant holds that there is another rationale the police may be able to rely on to search the passenger compartment of the car: the police can search if they have facts that make it "reasonable to believe the vehicle contains evidence of the offense of arrest." Here's my question: What level of certainty does the "reasonable to believe" standard require? Is that probable cause? Reasonable suspicion? Something else?

  At first blush, my thought was that "reasonable to believe" surely can't mean probable cause: Under the automobile exception, the police can search any part of a car that might store evidence if they have probable cause to believe that evidence is in the car. That's true without an arrest, and it's why the police almost never get a warrant to search a car. Notably, Justice Alito in his dissent assumes that "reason to believe" is different from probable cause. (Alito asks, "Why, for example, is the standard for this type of evidence-gathering search 'reason to believe' rather than probable cause?") But if it's not probable cause, what is it?

  Let's take a look. We start with the holding of Justice Stevens' majority opinion in Gant that adopted the "reasonable to believe" test:

[W]e hold that Belton does not authorize a vehicle search incident to a recent occupant’s arrest after the arrestee has been secured and cannot access the interior of the vehicle. Consistent with the holding in Thornton v. United States, 541 U. S. 615 (2004) , and following the suggestion in Justice Scalia’s opinion concurring in the judgment in that case, id., at 632, we also conclude that circumstances unique to the automobile context justify a search incident to arrest when it is reasonable to believe that evidence of the offense of arrest might be found in the vehicle.

The analysis section of the opinion offers this explanation:

Although it does not follow from Chimel, we also conclude that circumstances unique to the vehicle context justify a search incident to a lawful arrest when it is “reasonable to believe evidence relevant to the crime of arrest might be found in the vehicle.” Thornton,541 U. S., at 632 (Scalia, J., concurring in judgment). In many cases, as when a recent occupant is arrested for a traffic violation, there will be no reasonable basis to believe the vehicle contains relevant evidence. See, e.g., Atwater v. Lago Vista, 532 U. S. 318, 324 (2001); Knowles v. Iowa, 525 U. S. 113, 118 (1998). But in others, including Belton and Thornton, the offense of arrest will supply a basis for searching the passenger compartment of an arrestee’s vehicle and any containers therein. . . .

  So the test is straight from Justice Scalia's concurrence in Thornton v. United States. Let's turn to that next.

  Unfortunately, Justice Scalia's opinion is not particularly clear on what the "reasonable to believe" standard means. As I read the opinion, there are two potentially relevant passages. First, Scalia quotes a source that used a "reason to believe" formulation, an 1872 edition of Bishop's Criminal Law treatise, 1 J. Bishop, Criminal Procedure §211, p. 127 (2d ed. 1872), which Scalia quotes as follows:

"The officer who arrests a man on a criminal charge should consider the nature of the charge; and, if he finds about the prisoner’s person, or otherwise in his possession, either goods or moneys which there is reason to believe are connected with the supposed crime as its fruits, or as the instruments with which it was committed, or as directly furnishing evidence relating to the transaction, he may take the same, and hold them to be disposed of as the court may direct." Bishop, supra, §211, at 127.

(emphasis added). Although this formulation uses the "reason to believe" language, it seems to be referring to the relevance point ex post after the evidence is found, not how much certainty must exist ex ante before the search can occur. So that doesn't seem very helpful.

  The second passage is more directly useful. Scalia's opinion in Thornton suggests that the "reasonable to believe" standard was applied in United States v. Rabinowitz, 339 U.S. 56 (1950), an early search-incident-to-arrest case involving a business. Scalia explains that in Rabinowitz,

we did not treat the fact of arrest alone as sufficient, but upheld the search only after noting that it was “not general or exploratory for whatever might be turned up” but reflected a reasonable belief that evidence would be found. 339 U.S., at 62—63;

  If you look up Rabinowitz, however, it does not actually use the "reasonble belief" formulation. Here's the relevant passage from Rabinowitz:

In the instant case, the search was not general or exploratory for whatever might be turned up. Specificity was the mark of the search and seizure here. There was probable cause to believe that respondent was conducting his business illegally. The search was for stamps overprinted illegally, which were thought upon the most reliable information to be in the possession of and concealed by respondent in the very room where he was arrested, over which room he had immediate control, and in which he had been selling such stamps unlawfully. Harris v. United States, 331 U. S. 145, which has not been overruled, is ample authority for the more limited search here considered.

  Hmm, not so helpful. Harris v. United States, cited in Rabinowitz, isn't helpful for our purposes, either. Indeed, Harris states at page 154 that no "mere evidence" can be searched for incident to arrest, which presumably went out the window with Warden v. Hayden in 1967 (if not before).

  So maybe Justice Scalia in Thornton had some more general sense of what "reasonable to believe" means in Fourth Amendment law? I'm not aware of the Supreme Court using "reasonable to believe" elsewhere in Fourth Amendment law, but I am aware of three different lines of cases that have used "reason to believe." Now, it's not obvious to me that a "reason to believe" standard is the same as a "reasonable to believe" standard. You could have a reason to believe X, and yet countervailing considerations could make it unreasonable to believe X. But it seems more likely that they are meant to be the same; note that Justice Alito's dissent in Gant repeatedly refers to the majority's standard as a "reason to believe" standard, not a "reasonable to believe" standard. See also United States v. Gorman, 314 F.3d 1105 n.4 (9th Cir. 2002) (noting that the terms are often used interchangeably, at least in the Ninth Circuit).

  So perhaps the Fourth Amendment lines of cases using a "reason to believe" standard can reveal the answer.