- Metric: law — written in 1791
The Fourth Amendment is the original American privacy law — written in 1791, in direct response to the British general warrants that let colonial officers ransack any house they liked. The text guarantees the right to be secure in your "persons, houses, papers, and effects, against unreasonable searches and seizures." Two and a half centuries of court decisions have spelled out what that actually means: in most cases, police need a warrant signed by a judge before they can search your home, your car, your phone, or your belongings.
The exceptions are large and well-worn. Consent (the easiest one — and the one police lean on most). Plain view. Exigent circumstances (active emergency, fleeing suspect, evidence about to be destroyed). Search incident to a lawful arrest. The automobile exception, when there's probable cause to believe a vehicle contains evidence of a crime. Each has its own specific rules. The default — no warrant, no search — still holds when none of them apply, and your job at the moment of an encounter is to keep the consent exception off the table.