Kant sought to make morality as pure and disconnected as possible from the vagaries of circumstance and individual personality. Whether an action is right or wrong was to depend solely on the principle of action that lay behind it, and in particular on whether the agent could regard that principle as one that everyone ought to follow in all possible circumstances. Professor Williams ingeniously showed how hard it is for any such insular conception of morality to survive contact with the messiness of real life. He coined the term "moral luck" to mark a fact that is incomprehensible to Kantians, but which—once he had highlighted it—came to seem undeniable: that whether or not a person's behaviour emerges as good or bad can sometimes depend on pure chance. Soldiers know that a man can become a hero partly by accident. The ancient tragedians knew that he could be undone by fate, which comes to much the same thing.

I WAS BICYCLING down the main street in Oxford and getting ready to turn right. (This was England, where a right turn is a left turn.) In a hurry, I gave only a quick look back before pulling away from the curb. I didn't see the cyclist behind me, crashed into him, and sent him sprawling into the roadway.

The cyclist was lucky. If there had been a car coming he would have been killed, but there wasn't. He brushed off his clothes, accepted my apology, and continued down the street.

I too was lucky. If there had been a car coming I would have been guilty of carelessly killing someone, a serious moral wrong. As it was, I had been only harmlessly negligent - nothing to be proud of, but no great sin.

But wait. How can luck have the second effect? How can fortune affect the wrongness of what I did? …

Consider a feature of our legal system: it punishes attempted crimes less severely than successful ones.

This might make sense if the law's purpose were just to prevent crimes. Stiffer penalties for successful crimes dissuade people who've already tried a crime unsuccessfully from trying it again. But if punishment is a response to guilt, the law is puzzling: why should a gunman be thought less guilty if his victim wore a bullet-proof vest?

We believe, it seems in "moral luck," in luck that affects not only what happens but the moral quality of what we do.