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, pr«m. partie, III., tome 1, p. 202. Dict. des Sciences M«dicales, Hallucination, tome 20, p. 64. See Hallucination.