In February and March 1974, Dick experienced a series of visions and other inexplicable perceptual and cognitive phenomena.
For the rest of his life, Dick explored the philosophical implications and hypothesized about the origins of the experience, in a journal which eventually ran to hundreds of thousands of words.
Dick's leading hypothesis was that he had been contacted by a transcendental, mystical mind he called VALIS (vast active living intelligence system).
Dick soon began talking (in letters and interviews) about a third novel to complete a "VALIS Trilogy."
After Dick's death, several omnibus editions of the "VALIS Trilogy" were published, with his final mainstream novel The Transmigration of Timothy Archer.