In an ideal world, there are no crimes.
In a slightly less ideal world, every crime would be witnessed by at least one sober, independent witness with perfect eyesight and a photographic memory.
In our don't-get-me-started-just-how-not ideal world, we often only have the victim's word to go on.
This makes it difficult to establish the truth beyond reasonable doubt.
It's a particular problem with Domestic Violence cases where almost by definition the offence is committed out of the public glare.
In the bad old days there would be no chance of a conviction - "it's just your word against his" - which is why historically so many abuse, domestic violence and rape cases never even made it to court.
Nowadays things do seem to have improved. More such cases are coming to court and convictions are possible without in any way diluting the absolute commitment to "proof beyond reasonable doubt".
This can only happen though when the accused's story is so full of holes and so internally inconsistent that there's no way that anything they say can be believed.
It also helps the prosecutor make the case when the accused is arrogant and hostile. It's hard to be an angry smart-alec while keeping your pack of lies consistent under expert questioning. In one recent case, the prosecutor said that if you were to believe his story, the accused must have "the reflexes of Spiderman". The accused nodded and said yeah - twelve years in the army - reflexes of Spiderman. Hmmm.
Today was a strange day - I gave Spiderman a community penalty.