It isn’t just that hair and fingernails grow and are cut off; our entire physical substance is slowly changing. What we today call atoms and molecules pass through us, leaving us with continuity of form but transience of matter. (C. S. Lewis, summarizing this argument, offers an illustration: I am in that respect, he says, like a curve in a waterfall.)16 This argument is repeated by Thomas Aquinas a millennium after Origen and nearly a millennium before Lewis. It’s a good argument: as we now know, we change our entire physical kit,
