The right of the people to be secure in their persons, houses, papers, and effects against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath of affirmation, and particularly describing the place to be searched, and the person or thing to be seized.
Except, of course, for some notable exceptions:
First class mail may be opened without a warrant on less than probable cause. ... Automotive travelers may be stopped ...
near the border without individualized suspicion even if the stop is based largely on ethnicity ... and boats on inland waters with ready access to the sea may be hailed and boarded with no suspicion whatsoever.