Alien life: how would we know?

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. The third, which stresses the body's self-production by energy budgeting and self-equilibrating energy exchanges of some (necessary) complexity, excludes both strong A-life and living non-biochemical robots.