Reasonable suspicion v. probable cause...

"'The infringement on personal liberty of any 'sezure' of a person can only be 'reasonable' under the Fourth Amendment if we require the police to possess 'probable cause' before they seize him...'

The fears I voiced in Terry about the weakening of the Fourth Amendment have regrettably been borne out by subsequent events. Hopes that the suspicion test might be employed only in the pursuit of violent crime-- a limitation endorsed by some of its proponents-- have now been dashed, as it has been applied in narcotics investigations, in apprehension of 'illegal' aliens, and indeed has come to be viewed as a legal construct for the regulation of a general investigatory police power. The suspicion test has been warmly embraced by law enforcement forces and vigorously employed in the cause of crime detention. In criminal cases we see those for whom the initial intrusion led to the discovery of some wrongdoing. But the nature of the test permits the police to interfere as well with a multitude of law-abiding citizens, whose only transgression may be a nonconformist appearance or attitude. As one commentator has remarked: 'Police power exercised without probable cause is arbitrary. To say that the police may accost citizens at their whim and may detain them upon reasonable suspicion is to say, in reality, that the police may both accost and detain citizens at their whim.'"

Justice Douglas, concurring in United States v. Brignoni-Ponce, (1975) 422 U.S. 873.