One long-past innocent day, in my prefolly youth, I came upon a statement in an undistinguished textbook on psychiatry that, as when Kant read Hume, woke me forever from my garden-of-eden slumber. "The psychotic does not merely think he sees four blue bivalves with floppy wings wandering up the wall; he does see them. An hallucination is not, strictly speaking, manufactured in the brain; it is received by the brain, like any 'real' sense datum, and the patient act in response to this to-him-very-real perception of reality in as logical a way as we do to our sense data. In any way to suppose he only 'thinks he sees it' is to misunderstand totally the experience of psychosis."

"Drugs, Hallucinations, and the Quest for Reality" (1964) quoting an unknown psychiatric text, reprinted in The Shifting Realities of Philip K. Dick (1995) Lawrence Sutin, ed.