Ornelas v. United States, 517 U.S. 690, 7 (1996)

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Opinion of the Court

rules." Gates, supra, at 232. We have described reasonable suspicion simply as "a particularized and objective basis" for suspecting the person stopped of criminal activity, United States v. Cortez, 449 U. S. 411, 417-418 (1981), and probable cause to search as existing where the known facts and circumstances are sufficient to warrant a man of reasonable prudence in the belief that contraband or evidence of a crime will be found, see Brinegar, supra, at 175-176; Gates, supra, at 238. We have cautioned that these two legal principles are not "finely-tuned standards," comparable to the standards of proof beyond a reasonable doubt or of proof by a preponderance of the evidence. Gates, supra, at 235. They are instead fluid concepts that take their substantive content from the particular contexts in which the standards are being assessed. Gates, supra, at 232; Brinegar, supra, at 175 ("The standard of proof [for probable cause] is . . . correlative to what must be proved"); Ker v. California, 374 U. S. 23, 33 (1963) ("This Cour[t] [has a] long-established recognition that standards of reasonableness under the Fourth Amendment are not susceptible of Procrustean application"; "[e]ach case is to be decided on its own facts and circumstances" (internal quotation marks omitted)); Terry v. Ohio, 392 U. S., at 29 (the limitations imposed by the Fourth Amendment "will have to be developed in the concrete factual circumstances of individual cases").

The principal components of a determination of reasonable suspicion or probable cause will be the events which occurred leading up to the stop or search, and then the decision whether these historical facts, viewed from the standpoint of an objectively reasonable police officer, amount to reasonable suspicion or to probable cause. The first part of the analysis involves only a determination of historical facts, but the second is a mixed question of law and fact: "[T]he historical facts are admitted or established, the rule of law is undisputed, and the issue is whether the facts satisfy the [relevant] statutory [or constitutional] standard, or to put it another

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