Imagine you are in a room. There are two other men in the room with you, one rich and one poor. The rich man is holding a gun to the poor man's head, and asking you to tell him to pull the trigger.

Your natural reaction, I hope, would be to tell him to drop the gun. You do so, and the rich man's face falls. His gun doesn't.

"Ah", he says, "but this man is dangerous. He wants to hurt us." You look at the man, half-starved and dressed in rags. He doesn't look like he could pose a threat to anyone, let alone the rich man with the gun in his hand. "He doesn't look like a threat to me", you say. "It looks to me like you're the one posing a threat to him."

The rich man looks puzzled, and beads of sweat start to form on his brow. He mops them with a handkerchief, with his other hand. Suddenly, his eyes brighten. "He is a threat," he says, "he has a rocket launcher."

You look at the man. He doesn't seem to have a rocket launcher on him. "He doesn't look like he has a rocket launcher," you say. You remember that you know someone who is an expert at finding rocket launchers on people. You call him up, and he arrives.

The rocket launcher-finder begins searching the man, but he finds nothing. He continues to look, but the rich man, getting increasingly agitated, tells him to leave. He, confused, does.

"See?" the rich man says.

"Hang on," you say, "you haven't shown me any evidence that this man has a rocket launcher on him. Either show me the evidence, or put the gun down."

The rich man gulps nervously, his eyes flickering around the room. "Oh, and another thing," he begins, "this man and his friends are being tortured by my friend. I have to shoot this man so that I can save his friends."

This argument has you puzzled. Surely there must be ways of saving the man and his friends without having to kill the man. You decide you need more information to make a valid judgement. "This friend of yours — how is he torturing this man?"

The rich man is hesitant to answer, but your persist until finally he blurts out, "Well, he used to torture them with these knives I sold him. But I stopped selling him them a few years back."

You are beginning to feel very suspicious of this rich man. Nevertheless, you feel you need more information before making a decision. After all, human life is at stake. "This man and his friends — why don't they try and stop your friend from torturing them?"