from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
ILLUSION. A species of mania in which the sensibility of the nervous system
is altered, excited, weakened or perverted. The patient is deceived by the
false appearance of things, and his reason is not sufficiently active and
powerful to correct the error, and this last particular is what
distinguishes the sane from the insane. Illusions are not unfrequent in a
state of health, but reason corrects the errors and dissipates them. A
square tower seen from a distance may appear round, but on approaching it,
the error is corrected. A distant mountain may be taken for a cloud, but as
we approach, we discover the truth. To a person in the cabin of a vessel
under sail, the shore appears to move; but reflection and a closer
examination soon destroy this illusion. An insane individual is mistaken on
the qualities, connexions, and causes of the impressions he actually
receives, and he forms wrong judgments as to his internal and external
sensations; and his reason does not correct the error. 1 Beck's Med. Jur.
538; Esquirol, Maladies Mentales, prem. partie, III., tome 1, p. 202. Dict.
des Sciences Medicales, Hallucination, tome 20, p. 64. See Hallucination.