You want to ensure people can do it right 99 percent of time. When we have to fire one of our surgical trainees, it is never because they dont have the physical skills but because they dont have the moral skills - to practise and admit failure.

Sometimes we can offer a cure, sometimes only a salve, sometimes not even that. But whatever we can offer, our interventions, and the risks and sacrifices they entail, are justified only if they serve the larger aims of a person's life. When we forget that, the suffering we inflict can be barbaric. When we remember it the good we do can be breathtaking.

Our ideas of what our priorities are shift as we come face-to-face with some of the struggles.

In one study, old people assigned to a geriatrics team stayed independent for far longer, and were admitted to the hospital less.

And in stories, endings matter.

One of the reasons people might be fallible, why we might fail to do what we try to do isignorance, that we have a limited understanding of the laws of the world - the physical laws that govern the world and of all the particulars of the world upon which those laws work. And then there's ineptitude, meaning that the knowledge is available, but individuals fail to apply it correctly. The third source is "necessary fallibility." That is, we're never going to be omniscient, there is some knowledge that we will simply never achieve, and there are limits to what we will be able to do.

In psychology, there's something called the broken-leg problem. A statistical formula may be highly successful in predicting whether or not a person will go to a movie in the next week. But someone who knows that this person is laid up with a broken leg will beat the formula. No formula can take into account the infinite range of such exceptional events.