Medical insanity means “the accused’s consciousness of the bearing of his act on those affected by it”.
Legal insanity means “the accused’s consciousness in relation to him”.
Medical insanity deals with the person’s previous and present conduct and behaviour. Whereas legal insanity deals with the wrong-doer must be under unsoundness of mind at the time of the incidence, and he does not know its nature and affects.
The opinion of medical evidence, however, eminent he may be, must not be read as conclusive of the fact of insanity. In dealing with medical evidence it must always be remembered that their function is to assist, not to supersede the judge.
The medical witness states the existence, character and extent of the mental disease. The judge is to decide whether the disease made out comes within the legal conditions which justify an acquittal on ground of insanity.
Therefore, mere abnormality of mind or partial delusion, irresistible impulse or compulsive behaviour of a psychopath does not give protection under Sec. 84