concurring.
The opinion of the Court might be read to indicate that Terry v. Ohio, 392 U. S. 1 (1968), is an almost unique exception to a hard-and-fast standard of probable cause. As our prior cases hold, however, the key principle of the Fourth Amendment is reasonableness — the balancing of competing interests. E. g., Delaware v. Prouse, 440 U. S. 648, 653-654 (1979); Michigan v. Tyler, 436 U. S. 499, 506 (1978); Marshall v. Barlow’s, Inc., 436 U. S. 307, 321-322 (1978); United States v. Martinez-Fuerte, 428 U. S. 543, 555 (1976); United States v. Brignoni-Ponce, 422 U. S. 873, 878 (1975); Terry v. Ohio, supra, at 20-21; Camara v. Municipal Court, 387 U. S. 523, 536-537 (1967). But if courts and law enforcement officials are to have workable rules, see Rakas v. Illinois, 439 U. S. 128, 168 (1978) (dissenting opinion), this balancing must in large part be done on a categorical basis — not in an ad hoc, case-by-*220case fashion by individual police officers. Cf. Mincey v. Arizona, 437 U. S. 385, 394-395 (1978). On the other hand, the need for rules of general applicability precludes neither the recognition in particular cases of extraordinary private or public' interests, cf. Zurcher v. Stanford Daily, 436 U. S. 547, 564-565 (1978), nor the generic recognition of certain exceptions to the normal rule of probable cause where more flexibility is essential. Cf., e. g., Terry v. Ohio, supra. It is enough, for me, that the police conduct here is similar enough to an arrest that the normal level of probable cause is necessary before the interests of privacy and personal security must give way.