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It’s alive!
Cohen F., John Wiley & Sons, Inc., New York, NY, 1994. Type: Book (9780471008606)
Date Reviewed: Nov 1 1994

The burgeoning research domain of artificial life (alife) is generating a rich stream of publications, ranging from abstract mathematical analyses of emergent behavior to highly speculative works that are more fiction than science.  Cohen’s  contribution to this bibliography is written mostly at the popular level, with a single optional chapter that requires an elementary knowledge of first-order logic. Cohen’s claim to fame is his 1986 dissertation at the University of Southern California that introduced computer viruses, and the reader learns a great deal about viruses, but this book is not a narrow exposition of these critters. Nor does it pretend to survey the field with any completeness (as, for example, do Levy’s book [1] and the proceedings of the alife conferences). Instead, it uses viruses as a stepping stone to lead the reader through many of the fundamental concepts of  alife  and to argue that alife is a present reality that has already fundamentally changed human society.

Cohen’s book is a collection of fairly independent essays written in informal prose. Chapter 1 provides a broad overview of the book’s major ideas, and briefly introduces some of the technical examples (computer viruses, the Game of Life, and Corewar) that are developed in more detail in later chapters. Chapter 2 then goes to the heart of the matter to tackle the definition of life. After discussing the dictionary definition and puzzling over several problematic examples, Cohen offers the following definition: “A creature is alive in a given environment if and only if it is composed of living creatures, or it can produce an ‘offspring’ that is alive in that environment” (p. 21). Three features of this definition are central to the ideas discussed in the rest of the book. First, a creature can be described as alive only in reference to some specified environment, inviting explicit discussion of different environments and what can be considered alive in each. Second, the essential characteristic of life is reproduction (as opposed, for example, to motion, sentience, or metabolism). Third, an entity that cannot itself reproduce (for example, a mule) may be considered alive if its components (the mule’s cells) can reproduce. The definition succeeds in including traditional carbon-based life (including the problematic mule), but also includes fire (in a combustible environment), crystals (in a saturated solution), and social structures, all of which Cohen unhesitatingly accepts as alive.

Chapter 3, “Ecosystems,” builds on the definition’s emphasis on environment. Using the similarity between Turing machines and the operation of genes as a basis, the chapter argues that computers are a credible environment for life. Developing this foundation further, chapter 4 provides a  Turing  machine–based first-order formalization of the definition of life. The mathematical details are supported by a clear prose exposition.

Chapters 5, 6, and 7 give various concrete examples of computer-based life. Chapter 5, “Life--The Game,” goes beyond Conway’s classic implementation to discuss cellular automata in general. Chapter 6 focuses attention on benevolent viruses (or “living programs,” a term Cohen introduces to avoid the negative connotations of “virus”). These UNIX examples propagate themselves through file systems and over networks to distribute software, implement distributed databases, clean up old files, and gather material to a central repository. Chapter 7 discusses two classic examples of alife, Corewar and Tierra. The book includes a Macintosh diskette with UNIX source for Corewar and the various programs discussed in chapters 5 and 6.

After this hands-on interlude, chapters 8 and 9 return to more philosophical issues. Chapter 8, “Evolution,” revisits the issues of environments and ecologies that chapter 3 introduced. Chapter 9, “The Future,” speculates about the directions that alife may take. Cohen anticipates that nanotechnology will be the agency through which computer-based life interacts with the physical world. He argues that humans are already codependent on their computer systems, and anticipates the development of new hybrid human-computer life forms.

The book is an accessible introduction to the technical issues of alife. Written by a major researcher in the field, it understandably highlights his particular contribution (viruses), but uses that focus to make points that go well beyond the concerns of a virus expert. Though Cohen’s environmentally based definition of life admits some candidates that seem intuitively strange (fire and crystals), it is developed with commendable rigor, and sets a useful standard for others to emulate. Of particular interest are Cohen’s occasional short digressions on the theological implications of alife. From the standpoint of many religious traditions, all life on earth is artificial life, since it is created by the deity. Unlike other authors on alife who either ignore the parallel or engage in antitheistic polemics, Cohen responsibly teases out some of the parallels, not in support of any traditional religious scheme, but to set computer life in the broader context of human thought and experience.

Reviewer:  H. Van Dyke Parunak Review #: CR118256
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