Hallucinatory Apparitions

"The old doctrine of ‘ghosts’ regarded them as actual ‘spirits’ of the living or the dead, freed from the flesh or from the grave. This view, whatever else may be said for it, represents the simple philosophy of the savage, which may be correct or erroneous. About the time of the Reformation, writers, especially Protestant writers, preferred to look on apparitions as the work of deceitful devils, who masqueraded in the aspect of the dead or living, or made up phantasms out of ‘compressed air’. The common-sense of the eighteenth century dismissed all apparitions as ‘dreams’ or hoaxes, or illusions caused by real objects misinterpreted.

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Modern science, when it admits the possibility of occasional hallucinations in the sane and healthy, also admits, of course, the existence of apparitions. These, for our purposes, are hallucinatory appearances occurring in the experience of people healthy and sane. The difficulty begins when we ask whether these appearances ever have any provoking mental cause outside the minds of the people who experience them—any cause arising in the minds of others, alive or dead. This is a question which orthodox psychology does not approach, standing aside from any evidence which may be produced."

- Andrew Lang (1844 - 1912)
The Book of Dreams and Ghosts