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M'Naghten rule

(mĭk-nôtn)
n.
  1. An 1843 English rule used as a classic example of a test of criminal responsibility and stating that a defense based on a plea of insanity must prove that, at the time of the act, the accused individual was mentally ill, and was not aware of the nature and quality of the act, or, if aware, did not know the act was wrong.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.