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To recognize alien life, we would have to be clear about the defining criteria of ' life '. Metabolism -in other words, biochemical fine-tuning -is one of these criteria. Three senses of metabolism are distinguished. The weakest allows strong artificial life (A-life) : virtual creatures having physical existence in computer electronics, but not bodies, are classed as ' alive'. The second excludes strong A-life but allows that some non-biochemical A-life robots could be classed as alive. Thedoi:10.1017/s1473550403001496 fatcat:kmozvand6zen5cij6fnva4eum4