"A person who has an inmost conviction of a sensation actually perceived, when no external object capable of exciting such sensation is within reach of the senses, is in a state of hallucination" (Esquirol).

" By hallucinations are understood subjective sensory images which are projected outwardly and which in that way acquire objectivity and reality " (Griesinger).

" A hallucination is a perception without an object" (Ball).

These three definitions are essentially identical. That of Ball appears to us to be the best on account of its conciseness.

Hallucinations may affect any of the senses. There are therefore as many varieties of hallucinations as there are censes.

Some properties are common to all varieties of hallucinations, others are peculiar to certain varieties.