We were forever coming together and diverging, always in motion. Before I met him, I had developed a bad habit of ceding desire to others. I had put my body and mind too much at the disposal of other people, allowed them authority over me. I had become adept at exercising my will to negate my will. An ugly habit; it still fills me with shame.

But now I found myself 50 and free — free to claim myself. And fit. For most of my life, I hadn’t really noticed the privilege of good health. I didn’t go around on a daily basis thinking, “Jackpot! Another day of not being sick!” But time had increased my awareness of how many people live with injury and illness, how uncommon it was that I had never suffered anything more grievous than having my wisdom teeth out. It had come to seem an embarrassment of unearned riches.

Then there was the fact that two of my children were already out of the house and the youngest was in his last year of high school. Soon I wouldn’t be needed for day-to-day mothering. On top of that, I have an academic job, which means several months of paid discretionary time each summer. All of which added up to the recognition that I had a surplus ability to be more useful. In the weeks surrounding my 50th birthday, this sense of surplus, and the question of how to channel it, built in me like steam in an engine.

When I heard the radio story about becoming an altruistic donor, it seemed an answer. I folded the last pair of socks, went to the computer and looked up kidney donation. I was uncomfortable with the phrase “altruistic donor,” which implied a lack of self-interest, when it was perfectly clear to me that I had great self-interest: It would make me feel good to be of use in this way.