A virus meets three of the six criteria usually listed, fire meets two, a crystal also two, and a mule meets five while nobody doubts it is alive. Set the bar at four criteria and the virus falls out; set it at three and it gets in, so the verdict tracks the number we chose rather than the object itself. NASA's working definition leans on the capacity for Darwinian evolution, but that is a tool for doing research, not a settled conclusion. No definition commands agreement among scientists, and life may simply be a continuum.
A virus meets three of the six criteria usually listed, fire meets two, a crystal also two, and a mule meets five while nobody doubts it is alive. Set the bar at four criteria and the virus falls out; set it at three and it gets in, so the verdict tracks the number we chose rather than the object itself. NASA's working definition leans on the capacity for Darwinian evolution, but that is a tool for doing research, not a settled conclusion. No definition commands agreement among scientists, and life may simply be a continuum.
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