And that, in brief, is the modern doctrine of ghosts.
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, such as rats, cats, white posts, maniacs at large, sleep-walkers, thieves, and so forth.

Modern science, when it admits the possibility of occasional hallucinations in the sane and healthy, also admits, of course, the existence of apparitions.