Ornelas v. United States

The Court flatly stated “[A]rticulating precisely what ‘reasonable suspicion’ and ‘probable cause’ mean is not possible. They are commonsense, nontechnical conceptions that deal with ‘the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act (underline added).’” Therefore, these terms are not “not readily, or even usefully, reduced to a neat set of legal rules.”

The Court has described (though not defined) reasonable suspicion as “a particularized and objective basis” for suspecting the person stopped of criminal activity (quoting United States v. Cortez, 449 U.S. 411 (1981)). Probable cause has been described (not defined) as known facts and circumstances sufficient to warrant a person of reasonable prudence in the belief that contraband or evidence of a crime will be found. Each case must be determined on its own facts. “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 (underline added).”