The Fourth Amendment was designed not merely to protect against official intrusions whose social utility was less as measured by some `balancing test' than its intrusion on individual privacy; it was designed in addition to grant the individual a zone of privacy whose protections could be breached only where the `reasonable' requirements of the probable-cause standard were met. Moved by whatever momentary evil has aroused their fears, officials - perhaps even supported by a majority of citizens - may be tempted to conduct searches that sacrifice the liberty of each citizen to assuage the perceived evil. But the Fourth Amendment rests on the principle that a true balance between the individual and society depends on the recognition of `the right to be let alone - the most comprehensive of rights and the right most valued by civilized men.' Olmstead v. United States, 277 U.S. 438, 478 (1928) (Brandeis, J., dissenting).