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) —Philip K. Dick