Prisoners are persons whom most of us would rather not think about. Banished from everyday sight, they exist in a shadow world that only dimly enters our awareness. They are members of a "total institution" that controls their daily existence in a way that few of us can imagine. "[P]rison is a complex of physical arrangements and of measures, all wholly governmental, all wholly performed by agents of government, which determine the total existence of certain human beings (except perhaps in the realm of the spirit, and inevitably there as well) from sundown to sundown, sleeping, walking, speaking, silent, working, playing, viewing, eating, voiding, reading, alone, with others. . . ." It is thus easy to think of prisoners as members of a separate netherworld, driven by its own demands, ordered by its own customs, ruled by those whose claim to power rests on raw necessity. -- Justice William Brennan, dissenting in O’Lone v. Estate of Shabazz, 482 U.S. 342, 354-55 (1987).