The chapters in this section discuss contemporary scientific views about two issues: how the familiar forms of life that exist on Earth originated from nonliving material by physico-chemical processes, and how familiar life forms might differ from other forms of life that naturally exist. Our scientific knowledge of natural life is limited to current life on Earth, which includes microbes (true bacteria, bacteria-like archea, and protists, e.g., paramecia) as well as larger, more familiar fungi (e.g., mushrooms), plants, and animals. For better or worse, our paradigm of life is founded upon these organisms because they represent the only examples of life of which we can be certain. Our understanding of life in general, including the possibilities for artificial life and extraterrestrial life, is strongly influenced by what biologists and biochemists have discovered about life on Earth. Moreover, by asking specific biological questions, deeper philosophical ones inevitably emerge. The import of these chapters is thus not limited to the topic of this section. Taken together, these chapters bring important and sometimes underappreciated discoveries in biology and biochemistry to bear on some of the oldest questions about life.
Philosophical questions inevitably arise as soon as one starts probing the origin and extent of natural life. Both scientific projects raise general philosophical questions about life, such as the following: What presuppositions about life are embedded in different views about how life arose or what forms could exist? What weight should we place on those presuppositions?
As the discussions in Sections I and II underscore, natural life as we know it on Earth today is an extremely complex phenomenon, having distinctive functional characteristics at the abstract level and comprising specific chemical elements, molecules, and biochemical pathways at the physical level. Its complex, multi-leveled, interconnected character poses problems for biologists trying to generalize to all forms of life: Which features of our only unequivocal example of life are fundamental to all life? Artificial life (also known as “ALife”) is an interdisciplinary study that aims to circumvent this difficulty by artificially synthesizing lifelike processes both in isolation and together at different levels of analysis. This allows researchers to better explore questions such as whether the stuff that comprises life is essential to it. Underlying research in ALife is the view that one of the best ways to understand something is to learn how to make it. The term “artificial” refers to the fact that humans are involved in the construction process, and indicates that the results might be quite unlike natural forms of life. There is a lively debate among ALife researchers over what types of artificial life qualify as genuine examples of life, instead of mere simulations or models of life. The question is, when is a simulation or model of life complete enough to be the real thing?
Artificial life research can be subdivided into three broad categories, reflecting three different types of synthetic methods (Bedau 2003, 2007).
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.
Humans have long been puzzled about the nature of life—how living things are similar to and different from nonliving things, both natural and artificial, and whether the characteristics that are universal in familiar Earth life are genuinely essential to all possible forms of life. The chapters in this section provide classical historical perspectives on present day philosophical and scientific debates about life. These perspectives have an often underappreciated and sometimes even unrecognized influence on current philosophical and scientific thought. Cutting-edge contemporary ideas are sometimes not so novel after all! More importantly, however, sometimes the older debates, which typically focus on more general, and hence more fundamental, conceptual issues, can provide unexpected insights into present-day controversies.
This section begins with the writings of three intellectual giants: Aristotle, René Descartes, and Immanuel Kant. Best known today for their philosophical work, each also made important contributions to the development of modern science. Each holds a different view about the nature of life. As the remaining chapters in this book illustrate, the differences between them are still relevant today.
One central theme running from the writings of the ancient Greek philosopher Aristotle right up to the present is the idea that living things have distinctive functional characteristics. Aristotle also thought that living things are distinguished from inanimate objects by the ability to self-organize (develop from fertilized eggs) and maintain this self-organization against both internal and external perturbations.
This book is a collection of readings about the nature of life. The idea for it was born when we first met and discovered our mutual interest in the nature of life, a shared background in philosophy of science and philosophy of biology, and a complementary scientific expertise in the origin of life and astrobiology (Cleland) and artificial life and synthetic biology (Bedau). We both wanted to have a book like this, so we decided to compile it together. Our interests and orientation led us to include material in four general areas: (i) classical philosophical and scientific discussions about the nature of life, (ii) contemporary scientific and philosophical discussions of the origins of life, and of chemical possibilities for unfamiliar forms of natural life, (iii) discussions of contemporary artificial life creations, including not just computer simulations but also self-reproducing robots, protocells, and other synthetic biology constructions created in the wet lab, and (iv) attempts by contemporary scientists and philosophers to describe and explain the nature of life in its most general, non-Earthcentric, form. Producing this book confirmed for us the value of combining multiple perspectives on life.
We hope that this book will inform philosophers about the latest scientific advances and introduce scientists to subtle philosophical puzzles and problems, and thereby foster new, well-informed and thoughtful philosophical and scientific reflection about the nature of life. In our opinion, genuine progress in understanding life crucially depends upon combining both scientific and philosophical perspectives on life.
WHAT EXPLAINS THE PHENOMENA OF LIFE
Life seems to be one of the most basic kinds of actual natural phenomena. A bewildering variety of forms of life surrounds us, but we usually have no difficulty distinguishing the living from the nonliving. That flower, that mushroom, that worm, that bird are alive; that rock, that mountain, that river, that cloud are not. Just as any attempt to divide nature at its joints must account for mind and matter, so it must account for life.
Yet it is notoriously difficult to say what life is, exactly. Many have noted this (e.g., ref. 1); Farmer and Belin (ref. 2, p. 818) put the point this way:
There seems to be no single property that characterizes life. Any property that we assign to life is either too broad, so that it characterizes many nonliving systems as well, or too specific, so that we can find counter-examples that we intuitively feel to be alive, but that do not satisfy it.
The fact today is that we know of no set of individually necessary and jointly sufficient conditions for life.
Nevertheless, there is broad agreement that life forms share certain distinctive hallmarks. Various hallmarks are discussed in the literature, and Mayr's list is representative and influential:
All levels of living systems have an enormously complex and adaptive organization.
Living organisms are composed of a chemically unique set of macromolecules.
The important phenomena in living systems are predominantly qualitative, not quantitative.
All levels of living systems consist of highly variable groups of unique individuals.
The chapters in this section focus on the big question: What is life? This question cannot be answered by providing a list of things that are or were once alive. It is concerned with the very nature of life, with what qualifies something as a living thing; life in this sense is not contrasted with death but rather with nonlife. This section contains a diverse collection of views, but the authors all share the conviction that there is something special and fundamental about living systems, and hence that a general, unified explanation of life is in order. Not everyone shares this view. As the chapters in Section III by Sober and Keller illustrate, some researchers are skeptical about the wisdom of formulating general theories of life.
Running through this section is a disagreement over whether the stuff that composes life is essential to it. Some authors simply assume that this is the case, although they might also emphasize the importance of the way in which it is structured (spatially arranged). Others explicate life in terms of abstract organizational or functional properties that are independent of the material out of which it is composed. How one decides this issue has critical consequences for the possibility of strong artificial life (see Section III) as well as the possibilities of truly alien forms of natural life (see Section II).
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