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What is the definition of life? I remember a conference of the scientific elite that sought to answer that question. Is an enzyme alive? Is a virus alive? Is a cell alive? After many hours of launching promising balloons that defined life in a sentence, followed by equally conclusive punctures of these balloons, a solution seemed at hand: "The ability to reproduce--that is the essential characteristic of life," said one statesman of science. Everyone nodded …

…would be less harmful to them. Thus, the PICERAS principles seem to be necessary for the operation of a living system. Mechanisms to achieve such a system can be varied as long as they satisfy the thermodynamic and kinetic requirements. We have one example, life on Earth, showing how it can be done. It will be interesting to see whether a different, self-consistent set of mechanisms could yield a model with life as an outcome.