The signature moments in Dick's fiction occur when a character's seemingly stable reality turns inside out, revealing itself to be false. A man discovers that he's not a human being but a sophisticated robot. A television star finds himself in an alternate world where he's literally nobody; there's no record of his existence. An ordinary guy is told that the memory and identity he's had all his life were actually implanted not long ago and that he's really another man altogether. A man living in a sweet 1950's small town realizes that it is entirely artificial.

If some of those scenarios sound familiar, it's not surprising; filmmakers have based such movies as ''Blade Runner,'' ''Total Recall'' and ''Minority Report'' on Dick's fiction. But acknowledged adaptations aren't the only cinematic manifestations of Dick's obsessions. ''The Truman Show'' bears a suspicious resemblance to the novel ''Time Out of Joint,'' and some of the most arresting independent films of recent years involve characters whose realities collapse, stutter or slip from their fingers. Perhaps the writer-director Christopher Nolan never thought of Dick while creating ''Memento,'' but his hero's struggle to function with almost no short-term memory is quintessentially Dickian. The muddied boundaries between fantasy and fact in ''Mulholland Drive,'' the looped fate in the underrated ''Donnie Darko,'' the easy access sought to some other, better identity in ''Being John Malkovich'' all constitute territory that Dick knew well. The hugely popular movie ''The Matrix'' is classic Dick without actually being based on one of his works, and, really, who is Freddy Krueger of the ''Nightmare on Elm Street'' films if not a cruder manifestation of the ubiquitous and demonic Palmer Eldritch?

Dickian devices and themes -- implanted memories, commodified identities, simulacra -- haunt contemporary literary fiction as well. The naming of years after corporate sponsors in David Foster Wallace's ''Infinite Jest''; the downtrodden, stigmatized souls in George Saunders's futuristic short stories, with their degrading theme park jobs; the dream worlds Haruki Murakami's characters tumble into and out of -- all partake of Dick's peculiar mixture of wrenched ontology and underdog sympathies.

Of course these writers could as easily have been inspired by the world around them as by Dick's stories and novels; a great speculative writer always extrapolates from the material at hand. What's striking is how early Dick zeroed in on those ideas. In 1968 (1968!) he opened the novel ''Do Androids Dream of Electric Sheep?'' with a husband and wife bickering over the proper use of their ''Penfield mood organs,'' gizmos that allow them to fine-tune their personalities. ''My schedule for today lists a six-hour self-accusatory depression,'' the wife announces, much to her husband's annoyance. He suggests she set the mood organ to 888, ''the desire to watch TV, no matter what's on it.''

Aldous Huxley used the idea of chemical mood control even earlier, in ''Brave New World,'' but Dick took speculative fiction's rarefied thought experiments and integrated them into the humble fabric of everyday life. As weird as his work can be, it's always grounded in the lives of Willy Lomanesque working stiffs -- late on the rent, nagged by their wives and just trying to get by. In his own life, Dick alternated between the 1950's ideal of a nuclear family and a freer but chaotic demimonde; that tension between midcentury suburbia and our liberationist impulses preoccupies us still. Like Dick's characters, we take comfort in vicarious glamour. The bored and miserable Mars colonists in ''Stigmata'' spend all their time playing with Barbie-like dolls. Using a drug called Can-D, they can transport themselves into the dolls and briefly become gorgeous young people who drive Jaguars, revel in seaside trysts and otherwise savor a life in which it's ''always Saturday.''