To fully understand why this is a paradox or at least something of a mystery, consider this story. A few weeks after a young recruit begins his military service he receives an assault rifle. Turning the rifle over, he notices the slot that secures the bayonet. But when he asks his sergeant when they will receive bayonets, the sergeant replies that bayonets violate the rules of war and would not be issued.

Technically, this is incorrect. There is no prohibition against standard issue bayonets. There is, however, a long-standing ban on bayonets with serrated edges. I will discuss the reasons for this in a moment, but for now, it is important to consider the entire episode from the young soldier's point of view. Reflecting on the bayonet ban he asks, Why are the methods of warfare limited? Why can't I use any means necessary to defeat an enemy who is trying to kill me? This question has puzzled people for the longest time. How do we answer it?

There is not one answer to this question but two. First, we avoid certain weapons because our moral intuitions tell us that some weapons are unnecessary to disable an armed adversary or simply inhuman.