When an act, which would otherwise be a certain offence, is not that offence, by reason of the youth, the want of maturity of understanding, the unsoundness of mind or the intoxication of the person doing that act, or by reason of any misconception on the part of that person, every person has the same right of private defence against that act which he would have if the act were that offence.
Illustrations
(a) Z, a person of unsound mind, attempts to killA; Z is guilty of no offence. ButAhas the same right of private defence which he would have if Z were sane.
(b) A enters by night a house which he is legally entitled to enter. Z, in good faith, taking A for a house-breaker, attacks A. Here Z, by attacking A under this misconception, commits no offence. ButA has the same right of private defence against Z, which he would have if Z were not acting under that misconception.
Legal Commentary & Landmark Judgments: Section 36
Section 36 establishes that the right of private defence is available against the act of a person of unsound mind, a child, an intoxicated person, or a person acting under a mistake of fact, ensuring defensive protection regardless of the attacker's personal criminal immunity.
If your client had to push away or disarm a mentally disturbed neighbor or an intoxicated person who attacked them with a knife, cite Section 36 BNS: your client had the complete legal right to defend themselves notwithstanding the attacker's psychiatric history.