Suppose Boeing 747s grew on trees. They would first sprout as embryonic planes, the size of an acorn. Then they would grow until they reached full size, when they would plop off the trees, ready to fly. Suppose also that we knew how to feed and care for them, how to make minor repairs, and of course how to fly them. But let us suppose that all of this transpired at a very early stage in our scientific history, when we did not yet understand the physics or the engineering of flight: hence the phenomenon was a complete mystery to us. (To keep things simple, let us suppose that no other entity on earth could fly, only 747s.) And for the last ingredient in this fantasy, suppose that computers likewise grew on trees, and we knew how to use and fix them too.
Some of us would no doubt make a cult of flight, believing that it was a God-given gift. Others would say that flight was an emergent phenomenon, not explicable in terrestrial terms. Still others would set to work trying to explain flight in the same way that everything else (bridges, carriages, catapults, and ovens) was explained. And philosophers would become involved too, trying to define flight, and to argue that it can be subsumed under ordinary earthly principles (or not, depending on the philosopher's school of thought).
Now the aspiring explainers might go about it in two ways. They could (based on the limited knowledge that they already had about how to fly, fuel, and do minor repairs on the planes) attempt to build one from scratch. Let us suppose that this research programme was going extremely slowly: they had not yet even found a way to build something that could taxi along on the ground, let alone get off it. Let us call this bottom-up approach to understanding flight "artificial flight" (a-flight), because of course the planes that grew on trees would be cases of natural flight.
Now recall that there was another unlikely entity of arboreal provenance in this fantasy: the computer. Suppose that the second kind of a-flight researcher tried to come to an understanding of the principles of flight, not by trying to build a plane, but by trying to simulate one computationally. As both of these approaches are examples of a-flight, let us call the first "synthetic flight" and the second "virtual flight."
Now let us suppose that the v-flighters beat the s-flighters to it: They get their virtual 747 aloft while the s-flighters are still working on getting theirs to back onto a runway. And suppose the v-flight community gets very excited about what they have done: "It flies! It's aloft!" The press is called in, along with the general public. A curtain is parted to reveal a huge computer screen; portentous music is played as the image of the 747 appears, its jet motors getting louder and louder as it picks up speed on the runway, until it becomes airborne and climbs to 40,000 feet where it proceeds to do aerial tricks to the delight of the children before descending in graceful circles to make a 3-point landing to a roar of applause - until a child cries out: "But that was just a cartoon plane!" - at which the audience gasps and murmurs "He's right! There's no plane! Its just a hoax!"
Who would in fact be right? If the v-flighters claimed that their plane really flew, then of course we would all agree that that was not true: flying only happens in a real sky. It is obvious that the computer is not flying; the images merely look as if they were flying. But what if the v-flighters said: "Fine; if you don't believe our simulation has really captured the principles of flight, then watch!" And they set about building a 747 by applying the principles that had successfully generated virtual flight, and the first synthetic plane they build according to those principles indeed takes to the air, just as the virtual plane had done.
Would there be a volume on The Philosophy of Artificial Flight in order to sort these things out? I think not, and the reason is simple: flight is observable, so we are capable of distinguishing veridical flight from virtual flight. Life, in contrast, is not observable. (You can observe living things, but not the property of being alive.) So we are free to fancy that virtual creatures are really alive.
Chris Langton's and Thomas Ray's chapters in The Philosophy of Artificial Life give inspiring and impassioned accounts of progress in v-life, a field to which they are active contributors. They describe their remarkable simulations of virtual creatures that feed, mate, compete, mutate and reproduce for generation after generation. But both authors insist that it cannot be ruled out that these creatures are really alive.
Now it is conceivable that the real scenario for life will be like the fantasy scenario for flight, in that v-life manages to discover the vital principles more quickly than s-life does. Will that also serve to quicken its virtual creatures? Philosopher Elliot Sober's chapter gives the same negative answer that is implicit in my little allegory about flight. And philosopher Maggie Boden, a seasoned exegete of artificial intelligence, does an admirable job of integrating the diverse views of her contributors. But otherwise I must say that one learns considerably less from the chapters by philosophers than from those by the nonphilosophers. The philosophers tell us about Aristotle's views on life, which are interesting although their relevance is unclear; even less edifying are the views of Spencer and Dewey on life. Some of the contemporary philosophers venture their own views on life, but one does feel that these are fussy and ad hoc and very much following rather than leading the thought on the subject. And the three chapters in which nonphilosophers philosophise are both speculative and vague, one invoking Heidegger, one Ernst Nagel, and the third passing the buck to quantum mechanics.
There is also a chapter on something even more immaterial than virtual life: possible life (p-life, of which real life is merely a subset) and the very general constraints on it that are suggested by the theoretical work of Stuart Kauffman on complexity, order and evolution. A breath of real life is felt in the short and only chapter on what we might call "n-life" in this context. Biologist John Maynard Smith reminds us that studying natural selection and real organisms may still reveal more than a-life, whether synthetic or virtual. Then biologist David McFarland turns this admonition on its head, suggesting that real organisms may be better understood as cost-based robots.
All in all, the topic is fascinating and the volume thought-provoking. It leaves one eager to hear who will ultimately win in this game of reverse-engineering life: the computationalists, the roboticists or the naturalists.
Stevan Harnad is professor of psychology, University of Southampton.
The Philosophy of Artificial Life
Author - Margaret A. Boden
ISBN - 0 19 8751559 9 and 875154 0
Publisher - Oxford University Press
Price - £35.00 and £11.99
Pages - 405