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If you pit yourself against a virtual jet fighter in a video game, you are up against a small patch of artificial intelligence within the larger program. It will behave in ways that favour its own survival, and threaten that of your own representation in the game. But if you destroy it with a simulated missile, you don't feel sorry for it. However lifelike its exertions in the struggle for existence, nobody imagines it was ever alive.

Norns, the subjects of the Creatures range of computer games, are a different kettle of fish. Their google eyes and pointy ears make Furbies look world-weary; and their owners - who number a million, it is claimed - feel intensely sorry for Norns that suffer at human hands. Creature-lovers bombarded the author of a Creature-torturing website with hate mail, while setting up rehab and adoption agencies for his victims. "They may have been overreacting, but I was glad of the sentiment," observes Steve Grand, creator of Creatures. He doesn't think that people should discriminate against a life form just because it is artificial. His book rests on the proposition that artificial life is not simulated life - even though it exists only within computers - or pseudo-life, but real life. Creatures, he claims, "was probably the closest thing there has been to a new form of life on this planet in four billion years".

Close, but no cyanobacteria. Grand doesn't believe in starting from scratch. For him, nature is an accumulated stock of gadgets to be simulated, from enzymes and chemoreceptors to instincts and attention-directing mechanisms, by translating them from biology into engineering. Standing on the shoulders of several billion years of evolution, he ends up contemplating the prospect of artificial entities on a mental par with squirrels, which might be suitable for controlling traffic lights.

It's not surprising to encounter this oddly engaging vision after Grand's exposition of how to engineer life, with its oscillators and integrators and phase-locked loops. The common ground between artificial life and practical electronics seems obvious. But it's not exactly what one is led to expect by Grand's opening claim, that the book "tries to answer the question, 'How can we build a soul?'" Then again, this may be a tall order if you believe with Grand that "philosophy is the art of stating the obvious". It's certainly difficult in the face of his contention that everything is inevitable, but we need to behave as though we have free will in order for society to endure.

The question one is left with is the more immediate one: "Has Steve Grand built life?" His argument is that if you simulate basic biological systems and connect them together in a suitable way, they will produce effects that are not simulations, but real artificial life. This claim is both bold and impressive; though it's as well to bear in mind that Grand is pushing at an open door here, since we are biased. Humans are peculiarly ready to believe that things are alive, even such unpromisingly inanimate ones as hills.

There is also a question left lurking. Never mind how people treat artificial beings: how will they treat us? Grand is working on a robot glider with "the mind of an eagle". He accepts that such devices could have military uses, but insouciantly remarks that these "seem to involve protecting life more than destroying it". A silicon eagle would be a dream weapon for a state willing to inflict casualties but not to take them, and would care about them no more than the simulated pilot in a video game.

Marek Kohn's As We Know It: Coming to Terms with an Evolved Mind, is published by Granta.