Largely based on the ruling in R v. M’Naghten, §84 of the Indian Penal Code, 1860, and its jurisprudence, the defence of insanity continues to operate within anachronistic theories of the mind and its understanding in the law. This paper explores how the inconsistent interpretation and application of the test under §84, as a result of reliance on long discarded notions, has injected arbitrariness and vagueness into the jurisprudence. The lack of a uniform standard in turn impacts the burden on the defence even if such burden is to be discharged on a ‘preponderance of probabilities’. With courts inferring incapacity of the accused based on a host of factors, each of which may or may not be relevant, it becomes unclear how the defence must establish its plea. Ultimately, the paper concludes that resolving the issues outlined might well require rewording and updating the insanity defence in India.