Section 84: The 'Insanity' Defence That Always Fails

Oh man, I've been up for 3 hours now and I'm still stuck on Section 84. Why does this section even exist? 'A person is of unsound mind... if on account of unsoundness of mind, a person uses reasonable care in watching pendency of a state which he ought to watch for the protection of life of himself or of others.' In other words, you can kill someone but if you're insane, it's not your fault. Problem is, insanity is so subjective - how do you even prove it in court? It's like trying to explain a Bollywood drama to a non-Hindi speaker. In R v. Khan [1871], the court rejected the insanity plea because the accused knew what he was doing, he just chose not to stop. And that's the thing - 'unsoundness of mind' doesn't necessarily mean you can't control your actions.