Mere Christianity works because Lewis was a master at two rhetorical arts, which he combined fluently: argument and depiction. Indeed, his friend Austin Farrer emphasized the latter as his chief talent, and Lewis himself spoke, not only of creating Narnia in terms of "seeing pictures in his head," but of his entire writing career in this way. In the last months of his life, he explained to a friend why he was no longer generating new work. He was ill, but he was not old: only in his mid-60s. The situation was simple, he said: "The pictures have stopped."...