A long time ago, I took a yearlong course of premed anatomy and physiology. Our professor, Dr. Welton, was tall and bald, wore a white lab coat, and knew a hundred of our names after the first week. I adored him and often attended his office hours, demanding to help him file or dissect.

Once, I asked him about memory. He had devoted a few weeks to the structure and function of the brain, but had barely mentioned memory. “Why are some memories more vivid than others?” I asked. We were in his office, standing in shadow by his desk. At…