Our culture conditions us to perceive the world in ways consistent with its own teachings: we interpret the world through a system of common values based on the mores, religion, mythology, symbolism, and literature of our culture. In Crossing the Shadow-Line, Martin Bock explores the work of several writers—Christian and non-Christian—whose works are heretical in the sense that they explore ways of seeing and knowing the world that depart from the characteristic modes of perception of a predominantly Christian culture.

While the dream vision and similar visionary experiences have long been part of the Christian literary tradition, major writers of the Romantic period such as Samuel Taylor Coleridge and Thomas De Quincey used the imagery of opium vision in their poems and prose fantasies. These works are characterized by exaggerated sensation, an animistic world in which the landscape comes alive, and hallucinatory visions in which time and space are elastic. The extraordinary way narrators or characters see the world estranges them from their culture and they become, in effect, heretical visionaries.