The fourth case mentioned here, absence of external stimulation, would naturally raise the nervous structures to an exceptional pitch of excitability.

Such a condition would, moreover, prove favourable to hallucination by blurring the distinction between mental image and actual impression.
_Hallucinations of Normal Life._ In normal life, perfect hallucinations, in the strict sense as distinct from illusions, are comparatively rare.

Fully developed persistent hallucinations, as those of Nicolai, the Berlin bookseller, and of Mrs.
A----, the lady cited by Sir D.Brewster, in his _Letters on Natural Magic_, point to the presence of incipient nervous disorder.

In healthy life, on the other hand, while everybody is familiar with subjective sensations such as flying spots, phosphenes, ringing in the ears, few fall into the error of seeing or hearing distinct recognizable objects in the absence of all external impressions.

In the lives of eminent men we read of such phenomena as very occasional events.