As the text indicates and we have repeatedly affirmed, “the ultimate touchstone of the Fourth Amendment is ‘reasonableness.'” Riley v. California, 573 U. S. ___, ___ (2014) (slip op., at 5) (some internal quotation marks omitted). To be reasonable is not to be perfect, and so the Fourth Amendment allows for some mistakes on the part of government officials, giving them “fair leeway for enforcing the law in the community’s protection.” Brinegar v. United States, 338 U. S. 160, 176 (1949). . . .Reasonable suspicion arises from the combination of an officer’s understanding of the facts and his understanding of the relevant law. The officer may be reasonably mistaken on either ground. Whether the facts turn out to be not what was thought, or the law turns out to be not what was thought, the result is the same: the facts are outside the scope of the law. There is no reason, under the text of the Fourth Amendment or our precedents, why this same result should be acceptable when reached by way of a reasonable mistake of fact, but not when reached by way of a similarly reasonable mistake of law.
[T]he inquiry is not as forgiving as the one employed in the distinct context of deciding whether an officer is entitled to qualified immunity for a constitutional or statutory violation. Thus, an officer can gain no Fourth Amendment advantage through a sloppy study of the laws he is duty-bound to enforce.
[T]he government cannot defend an officer’s mistaken legal interpretation on the ground that the officer was unaware of or untrained in the law. And it means that, contrary to the dissenting opinion in the court below, an officer’s reliance on “an incorrect memo or training program from the police department” makes no difference to the analysis. 366 N. C. 271, 284, 737 S. E. 2d 351, 360 (2012) (Hudson, J., dissenting). Those considerations pertain to the officer’s subjective understanding of the law and thus cannot help to justify a seizure.. . ..
Justice Story’s opinion in The Friendship, 9 F. Cas. 825, 826 (No. 5,125) (CC Mass. 1812) (cited ante, at 7), suggests the appropriate standard for deciding when a legal error can support a seizure: when an officer takes a reasonable view of a “vexata questio” on which different judges “h[o]ld opposite opinions.” See Brief for United States as Amicus Curiae 26 (invoking that language). Or to make the same point without the Latin, the test is satisfied when the law at issue is “so doubtful in construction” that a reasonable judge could agree with the officer’s view. The Friendship, 9 F. Cas., at 826.A court tasked with deciding whether an officer’s mistake of law can support a seizure thus faces a straightforward question of statutory construction. If the statute is genuinely ambiguous, such that overturning the officer’s judgment requires hard interpretive work, then the officer has made a reasonable mistake. But if not, not. As the Solicitor General made the point at oral argument, the statute must pose a “really difficult” or “very hard question of statutory interpretation.” Tr. of Oral Arg. 50. And indeed, both North Carolina and the Solicitor General agreed that such cases will be “exceedingly rare.” Brief for Respondent 17; Tr. of Oral Arg. 48.
Heien is correct that in a number of decisions we have looked to the reasonableness of an officer’s legal error in the course of considering the appropriate remedy for a constitutional violation, instead of whether there was a violation at all. See, e.g., Davis v. United States, 564 U. S.___, ___ (2011) (slip op., at 11) (exclusionary rule); Illinois v. Krull, 480 U. S. 340, 359–360 (1987) (exclusionary rule); Wilson v. Layne, 526 U. S. 603, 615 (1999) (qualified immunity); Anderson v. Creighton, 483 U. S. 635, 641 (1987) (qualified immunity). In those cases, however, we had already found or assumed a Fourth Amendment violation. An officer’s mistaken view that the conduct at issue did not give rise to such a violation—no matter how reasonable—could not change that ultimate conclusion. See Brief for Respondent 29–31; Brief for United States as Amicus Curiae 30, n. 3. Any consideration of the reasonableness of an officer’s mistake was therefore limited to the separate matter of remedy.Here, by contrast, the mistake of law relates to the antecedent question of whether it was reasonable for an officer to suspect that the defendant’s conduct was illegal. If so, there was no violation of the Fourth Amendment in the first place.