In drawing back the curtain on what compels humans to "keep on keeping on," Cave engages the reader in a number of mind-bending thought experiments. He teases out the implications of each immortality gambit, asking, for example, how long a person would live if they did manage to acquire a perfectly disease-free body. Or what would happen if a super-being tried to round up the atomic constituents of all who've died in order to resurrect them. Or what our loved ones would really be doing in heaven if it does exist. Or what part of us actually lives in a work of art, and how long that work of art can survive.

Toward the book's end, we're confronted with a series of brain-rattling questions: What would happen if tomorrow humanity discovered that there is no life but this one? Would people continue to care about their favorite sports team, please their boss, vie for the title of Year's Best Salesman? Would three-hundred-year projects still get started?

Immortality is a deeply satisfying book, as optimistic about the human condition as it is insightful about the true arc of history.