Although hallucinations are so common and universally recognised as an indication of insanity (Esquirol reckons, probably truly if we count every stage of every case, that they occur in 80 per cent. of cases of mental alienation, and we all know that even with the general public there is no proof of aberration more convincing), yet there are many points connected with the study of hallucination which are worthy of more attention than they generally receive.