Greg Egan is science fiction's most rigorous explorer of ideas at the boundary of physics, computation, and identity, and this collection samples his distinctive approach. The stories here, gathered from various publications, tackle questions that less ambitious writers would avoid: What happens when you can copy a person's mind? What would it mean to live in a universe with different physical laws? How should we treat artificial intelligences that we create and can delete? Egan, an Australian writer who protects his privacy so thoroughly that no photographs of him have been published, writes hard science fiction that remains hard even when the science becomes speculative. His characters grapple with implications that their situations demand, working through consequences that stories with softer premises would elide. The title story imagines a billionaire who creates universes as computational simulations, evolving intelligent life to solve his problems, raising questions about our responsibilities to beings we create. Other stories explore the mathematics of consciousness, the experience of uploaded minds, and the possibility of physics radically different from our own. Egan's prose is clear and unadorned, letting ideas carry the weight. Readers who want their science fiction to take science seriously, who enjoy having their assumptions challenged, will find Egan essential.