This book is a collection of classic and contemporary readings by philosophers and scientists on the nature of life. Philosophers have pondered the question “what is life?” since at least the time of the ancient Greek philosopher Aristotle. In recent years the question has taken on increasing scientific importance. Molecular biologists and biochemists investigating the origin of life or trying to synthesize chemical life in the laboratory from basic molecular building blocks want to know at what stage an ensemble of nonliving molecules turns into a primitive living thing. Astrobiologists charged with designing instrument packages for spacecraft to detect extraterrestrial life struggle with the question of which characteristics of familiar Earth life (metabolism? reproduction? Darwinian evolution? carbon-based chemistry?) are universal indicators of life. Even computer scientists find themselves mired in questions about the nature of life when they speculate whether lifelike systems constructed of software (purely informational or digital structures) or hardware (metal, plastic, and silicon) could ever literally be alive. Many of these pressing questions are notable for their lack of obvious scientific solutions; one cannot answer them merely by performing more experiments or constructing additional lifelike systems.