It is a fundamental principle of criminal jurisprudence that an accused is presumed to be innocent and, therefore, the burden lies on the prosecution to prove the guilt of the accused beyond reasonable doubt. The prosecution, therefore, in a case of homicide shall prove beyond reasonable doubt that the accused caused death with the requisite intention described in section 299 of the Indian Penal Code. This general burden never shifts and it always rests on the prosecution. But, as section 84 of the Indian Penal Code provides that nothing is an offence if the accused at the time of doing that act, by reason of unsoundness of mind was incapable of knowing the nature of his act or what he was doing was either wrong or contrary to law. This being an exception, under section 105 of the Evidence Act the burden of proving the existence of circumstances bringing the case within the said exception lies on the accused; and the court shall presume the absence of such circumstances. Under section 105 of the Evidence Act, read with the definition of “shall presume” in section 4 thereof, the court shall regard the absence of such circumstances as proved unless, after considering the matters before it, it believes that said circumstances existed or their existence was so probable that a prudent man ought, under the circumstances of the particular case, to act upon the supposition that they did exist. To put it in other words, the accused will have to rebut the presumption that such circumstances did not exist, by placing material before the court sufficient to make it consider the existence of the said circumstances so probable that a prudent man would act upon them. The accused has to satisfy the standard of a “prudent man”. If the material placed before the court, such as, oral and documentary evidence, presumptions, admissions or even the prosecution evidence, satisfies the test of “prudent man”, the accused will have discharged his burden. The evidence so placed may not be sufficient to discharge the burden under section 105 of the Evidence Act, but it may raise a reasonable doubt in the mind of a Judge as regards one or other of the necessary ingredients of the offence itself. It may, for instance, raise a reasonable doubt in the mind of the Judge whether the accused had the requisite intention laid down in section 299 of the Indian Penal Code. If the Judge has such reasonable doubt, he has to acquit the accused, for in that event the prosecution will have failed to prove conclusively the guilt of the accused. There is no conflict between the general burden, which is always on the prosecution and which never shifts, and the special burden that rests on the accused to make out his defence of insanity.
[Extract from Dahyabhai Chhaganbhai Thakker v. State of Gujarat, AIR 1964 SC 1563, para 5].