On February 20, 1974, while recovering from the effects of sodium pentothal administered for the extraction of an impacted wisdom tooth, Dick received a home delivery of Darvon from a young woman. When he opened the door, he was struck by the dark-haired girl's beauty, and was especially drawn to her golden necklace. He asked her about its curious fish-shaped design. As she was leaving, she replied: "This is a sign used by the early Christians." Dick called the symbol the "vesicle pisces". This name seems to have been based on his conflation of two related symbols, the Christian ichthys symbol (two intersecting arcs delineating a fish in profile), which the woman was wearing, and the vesica piscis.
Dick recounted that as the sun glinted off the gold pendant, the reflection caused the generation of a "pink beam" of light that mesmerized him. He came to believe the beam imparted wisdom and clairvoyance, and also believed it to be intelligent. On one occasion, he was startled by a separate recurrence of the pink beam, which imparted the information that his infant son was ill. The Dicks rushed the child to the hospital, where the illness was confirmed by professional diagnosis.
After the woman's departure, Dick began experiencing strange hallucinations. Although initially attributing them to side effects from medication, he considered this explanation implausible after weeks of continued hallucination. He told Charles Platt:
"I experienced an invasion of my mind by a transcendentally rational mind, as if I had been insane all my life and suddenly I had become sane."
Throughout February and March 1974, Dick experienced a series of hallucinations which he referred to as "2-3-74", shorthand for February–March 1974. Aside from the "pink beam", he described the initial hallucinations as geometric patterns, and, occasionally, brief pictures of Jesus and ancient Rome. As the hallucinations increased in duration and frequency, Dick claimed he began to live two parallel lives—one as himself, "Philip K. Dick", and one as "Thomas", a Christian persecuted by Romans in the first century AD. He referred to the "transcendentally rational mind" as "Zebra", "God" and "VALIS" (an acronym for Vast Active Living Intelligence System). He wrote about the experiences, first in the semi-autobiographical novel Radio Free Albemuth, then in VALIS, The Divine Invasion, The Transmigration of Timothy Archer and the unfinished The Owl in Daylight (the VALIS trilogy).
In 1974, Dick wrote a letter to the FBI, accusing various people, including University of California, San Diego professor Fredric Jameson, of being foreign agents of Warsaw Pact powers. He also wrote that Stanisław Lem was probably a false name used by a composite committee operating on orders of the Communist party to gain control over public opinion.
At one point, Dick felt he had been taken over by the spirit of the prophet Elijah. He believed that an episode in his novel Flow My Tears, the Policeman Said was a detailed retelling of a biblical story from the Book of Acts, which he had never read. He documented and discussed his experiences and faith in a private journal he called his "exegesis", portions of which were later published as The Exegesis of Philip K. Dick. The last novel he wrote was The Transmigration of Timothy Archer; it was published shortly after his death in 1982.