But I do not think it can legitimately be applied to all visions of this class.
The point is of some interest and worthy of a moment's thought even though it involves a digression from the main topic.
The essence of hallucination is that it should have a purely subjective origin and be unfounded on objective reality.
If I were to look round and find my sofa occupied by three green cassowaries playing nap I should, I think, be justified in assuming that I was the victim of an hallucination having no foundation in objective fact. It would, presumably, have arisen from a simultaneous excitation of the memory centres associated with the game of nap, cassowaries, the number three, and the sensation of greenness, occasioned, more or less fortuitously, by over-work or alcoholic excess.
On the other hand if I were to see the figure of an old man with a long white beard, one front tooth missing, shaggy eyebrows, black velvet smoking jacket, gold watch and chain, and so forth and were subsequently to find that such a person, answering the description in every detail, and previously entirely unknown to me, had really once lived, or was still living, then the view that this vision was the result of pure hallucination, would be untenable.
The probabilities against any chance stimulation of memory centres giving rise to precisely that combination of characteristics, are immeasureably large.
In such cases—and they are by no means unknown—we must attribute some degree of objectivity to the origin of the vision.
This is of importance in view of the tendency in some quarters to dismiss all such visions as purely hallucinatory.