Integrating both scientific and philosophical perspectives, this book provides an informed analysis of the challenges of formulating a universal theory of life. Among the issues discussed are crucial differences between definitions and scientific theories and, in the context of examples from the history of science, how successful general theories develop. The central problem discussed is two-fold: first, our understanding of life is still tacitly wedded to an antiquated Aristotelian framework for biology; and second, there are compelling reasons for considering that familiar Earth life, which descends from a last universal common ancestor, is unrepresentative. What is needed are examples of life as we don't know it. Potential sources are evaluated, including artificial life, extraterrestrial life, and a shadow biosphere right here on Earth, and a novel strategy for searching for unfamiliar life in the absence of a definition or general theory is developed. The book is a valuable resource for graduate students and researchers studying the nature, origins, and extent of life in the universe.

'What is life? What universal principles apply to any biosphere? Our efforts to answer these deep questions are stymied because of our biased, Earth-bound perspective with only one kind of (known) life. In a book rich with original ideas and lucid insights, science philosopher Carol E. Cleland considers life from the perspective of what we don’t know - the limitations, hidden biases, sloppy definitions, and false assumptions that may lead us astray. From 'shadow biospheres' on Earth, to artificial life in the lab, to alien lifeforms in distant galaxies, Cleland expands our minds and leads us to rethink what we thought we knew.'

Robert Hazen - Carnegie Institution for Science, Washington

'Searching for life elsewhere in our solar system or beyond is at the forefront of science today, due to recent discoveries about terrestrial life, planetary environments, and planets around other stars. We can’t extrapolate from our single example of life on Earth, which all share common biochemistry and are descended from a common ancestor, as to what the characteristics of life elsewhere in the universe might be. Given these uncertainties, how can we identify something as alive? What does it mean to be living? What is life? Carol E. Cleland takes a philosophy of science approach to what constitutes life, integrating it with biology in a planetary context. She has made a valuable contribution to our understanding of the nature of life and of how to search for it, both on Earth and elsewhere.'

Bruce Jakosky - University Of Colorado

'An essential read for anyone interested in the nature of life and its origins. Cleland’s philosophical outlook means that she approaches the subject from a fresh perspective, framing important questions rarely discussed by scientists: what does it mean to try to define life, water, or anything else? How likely is it that life will be found on other planets or satellites, such as Titan, with conditions very different from those on Earth? Can we be sure that life as we know it is the only sort of life possible? Moreover, can we be sure that life very different from the life we know doesn’t already exist unrecognized on Earth itself? This book discusses these points in a provocative way that forces readers to examine some of their cherished beliefs that they thought were self-evident.'

Athel Cornish-Bowden - Centre National de la Recherche Scientifique, France

‘This book focuses on the search for a universal theory of life and portrays viewpoints of both the philosopher and the scientist. This makes the book unique, providing at times astonishing insights.’

Dirk Schulze-Makuch Source: Basic and Applied Ecology

‘Cleland … encourages readers to think more broadly about the definition of life, avoiding the 'neo-Aristotelian' narrowness of current investigations. This work is indeed scholarly, with over 20 pages of references, and interdisciplinary … three different approaches are philosophically presented to enable readers to conceptualize searching for ‘life as we don’t know it’.’

T. Barker Source: Choice