Are You a Platonist or an Aristotelian?

Want to stay current with Arthur’s writing? Sign up to get an email every time a new column comes out.

Growing up, my older brother was a good student, interested in science. We shared a bedroom, so I benefited from his knowledge at night as we lay in our beds and he regaled me with facts of all kinds, with specializations on such topics as the behavior of dinosaurs and the age of volcanoes. One scientific idea he talked about particularly stimulated my imagination—and has stayed with me to this day.

Throughout our bodies, our cells die and regenerate over and over again. Altogether, he told me, the cells in our bodies get turned over at least once every seven years. It turns out that this isn’t precisely right: Different cells regenerate at , and a small number of cells in the heart and in the brain will be the same when I die as when I was born. But for the most part, the seven-year rule is true, which leads to the