A master science writer explores the definition of life. After reviewing the history of the concept of life, Zimmer recounts his global travels interviewing scientists who have made dazzling discoveries. However, when it comes to defining life itself, they cannot improve on the Supreme Court’s view of pornography: They’ll know it when they see it. The author quotes Frances Westall and André Brack, who wrote in 2018 “that there are as many definitions of life as there are people trying to define it.” In the end, writes Zimmer, “to be alive is not to be dead.” Despite the countless possible definitions, most biologists agree on a few hallmarks: Every creature that lives must metabolize (eat and digest), gather information about the surroundings, maintain homeostasis (keep the internal environment steady), reproduce, and evolve. Zimmer gives ample space to nitpickers who point out exceptions, and a few chapters record interviews with scientists exploring each of these hallmarks. None answer the author’s big question, but readers will not complain because Zimmer is such an engaging communicator. Confronting a possibly unanswerable question, the author explores its history, an eye-opening review of three centuries of research by intensely curious, obsessive, often obscure scientists who contributed to many revelations about the amazing attributes of life, when they weren’t deluded—e.g., 18th-century vitalists, who believed that “life contained a vital force that endowed matter with self-directed motion and the power to generate new complex bodies.” Veteran readers will not be surprised that Zimmer’s conclusion describes efforts to create life in the laboratory, a process whose possibility was suggested a century ago and whose first and many subsequent attempts produced headlines and increasingly complex but lifeless organic material. The author leaves no doubt that this century’s dazzling advances in genetics, biochemistry, DNA and RNA manipulation, and lipid membrane formation will bring home the bacon. An ingenious case that the answers to life’s secrets are on the horizon.