Hallucination and Reality

Louis West:

Imagine a man standing at a closed glass window opposite his fireplace, looking out at his garden in the sunset. He is so absorbed by the view of the outside world that he fails to visualize the interior of the room at all. As it becomes darker outside, however, images of the objects in the room behind him can be seen reflected dimly in the window glass. For a time he may see either the garden (if he gazes into the distance) or the reflection of the room's interior (if he focuses on the glass a few inches from his face). Night falls, but still the fire burns brightly in the fireplace and illuminates the room. The watcher now sees in the glass a vivid reflection of the interior of the room behind him, which appears to be outside the window. This illusion becomes dimmer as the fire dies down, and, finally, when it is dark both outside and within, nothing more is seen. If the fire flares up from time to time, the visions in the glass reappear.

In an analogous way, hallucinatory experiences such those of normal dreams occur when the "daylight" (sensory input) is reduced while the "interior illumination" (general level of brain arousal) remains "bright", and images originating within the "rooms" of our brains may be perceived (hallucinated) as though they came from outside the "windows" of our senses. . . .

It appears that all human behaivor and experience (normal as well as abnormal) is well attended by illusory and hallucinatory phenomena. While the relationship of these phenomena to mental illness has been well documented, their role in everyday life has perhaps not been considered enough. Greater understanding of illusions and hallucinations among normal people may provide explanations for experience otherwise relegated to the uncanny, "extrasensory," or supernatural.

--Entry on "Hallucinations" in the 15th Encyclopedia Britannica