The Book of Dreams and Ghosts

This is a good example of a solitary hallucination in the experience of a very cool-headed observer.  The causes of such experiences are still a mystery to science.  Even people who believe in “mental telegraphy,” say when a distant person, at death or in any other crisis, impresses himself as present on the senses of a friend, cannot account for an experience like that of the diplomatist, an experience not very uncommon, and little noticed except when it happens to coincide with some remarkable event. {56b}  Nor are such hallucinations of an origin easily detected, like those of delirium, insanity, intoxication, grief, anxiety, or remorse.  We can only suppose that a past impression of the aspect of a friend is recalled by some association of ideas so vividly that (though we are not consciously thinking of him) we conceive the friend to be actually present in the body when he is absent.