For years, fans of the Batman comics have puzzled over a mystery at the heart of the series: why doesn't Batman just kill his arch-nemesis, the murderous Joker?

The two have engaged in a prolonged game of cat-and-mouse. The Joker commits a crime, Batman catches him, the Joker is locked up, and then invariably escapes.

Wouldn't all this be much simpler if Batman just killed the Joker? What's stopping him?

Enter philosopher Immanuel Kant and the deontological theory of ethics.

At least, that's how the discussion progresses in a growing number of philosophy classes in the US