Seances occur only in darkened rooms, where the ghostly visitors can be seen dimly at best. If we turn up the lights a little, so we have a chance to see what’s going on, the spirits vanish. They’re shy, we’re told, and some of us believe it. In twentieth-century parapsychology laboratories, there is the ‘observer effect’: those described as gifted psychics find that their powers diminish markedly whenever sceptics arrive, and disappear altogether in the presence of a conjuror as skilled as James Randi. What they need is darkness and gullibility.

— Carl Sagan

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