maudsley's definition of hallucinations and illusions

   In 1887 the British physician and professor of medical jurisprudence Henry Maudsley (1835-1918) defined hallucinations and illusions as follows: "By hallucination is meant such a false perception of sense as a person has when he sees, hears, touches, or otherwise apprehends as external, that which has no existence at all outside his consciousness, no objective basis - sees a person where there is no person, hears a voice where there is no voice. It is the creation of a fitting object of sense as cause of a special sensation where no such object is; and it takes place in accordance with the well-known physiological law that it is possible, by stimulating artificially the nerve-centres of perception, to produce the same kind of perception, and sometimes in quite as vivid degree, as the natural stimulus of the proper external object would occasion. When there is an external object which excites the perception, but the nature of it is mistaken - far the most common case - it is usual and useful to describe the effect as illusion, although it is not possible in nature to draw a distinct line always between hallucination and illusion."

   References

   Maudsley, H. (1887). Natural causes and supernatural seemings. Second edition. London: Kegan, Paul, Trench & Co.