ast summer, an elderly man left his house with some errands to run. He would take the dog to the vet, and, on the way, he would pick up his hearing aid, which he'd left in a shop for repair. But a few minutes later, the man, a distinguished gentleman who liked to go dancing on Saturday nights, stood in the shop insisting that the clerk should put the hearing aid into his dog.

Nobody knows why this happened. Maybe his pills for the pain in his back were to blame. Or maybe his confusion was another penalty for growing old. There had been so many penalties. Giving up golf. Selling his farm. And worst of all, losing his wife. Maybe the glitches annoying him lately — those pesky little lapses of memory, those ever-more-frequent slips of the tongue, those wobbles each time he rose from his lounger — had been hinting of this all along.

Very soon, his mind — which had always been geared to his work and routine — began to take a poetic turn. Image connected with image — old ones and new ones reshuffled through time. In the hospital, he awoke each morning alarmed to find himself tethered to machines. "Let me up," he said. "I have to feed my cows." For those last few weeks, his children were youngsters in need of instruction, his cows had come up to the fence for their hay, and his wife had stepped out for a minute or two. He could even hear her voice, just there, down the hall — as though his brain, in his last days, had cut its losses and wired around the pain.

Of all the mysteries, these are the deepest and closest to home. How does a mind come to be? How does it blossom and thrive? How does it cope with injury, aging, addiction, disease? And how does it end? In the landscape of the human brain, there are one hundred billion neurons. If you could line the neurons up and walk them, you'd walk 600 miles. So the science of the brain is vast, but it is not distant. It is close as the sound of her laughter, the smell of the hay.