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Table 1 Several examples illustrating the “splitting” definition of the life concept

From: The essence of life

First, do viruses belong to life? This is a classic question reflecting our blurry concept concerning life. Obviously, a virus is not a self-sustaining chemical system – e.g., no metabolism when outside a host cell; however, when you say it is not life, you may look back and feel that it is indeed something quite different from the non-life background. Here we can make it clearer: the “form” of a virus is capable of undergoing Darwinian evolution, thus being a life form; a virus itself, as an individual, does not constitute a living entity (we may call it a “life entity” instead, see text). Second, is an old rabbit, no longer fertile, belongs to life? This is another classic question reflecting our blurry concept concerning life. Obviously, such an old rabbit no longer participates in Darwinian evolution, which characterizes life, but also obviously, it has not died – it is something quite different from the non-life background (surely, most of us would like to say it is alive). Here we can make it clearer: the form of the rabbit is no doubt capable of undergoing Darwinian evolution, thus being a life form; the old rabbit is no doubt still a living entity that self-sustains – though it will no longer engage into further Darwinian evolution. Note that as mentioned in the expression, a living entity results from Darwinian evolution, but, as an individual, only “might” engage into further Darwinian evolution. Third, somewhat oddly, even if a rabbit is a fertile one, there could still be doubts on whether it belongs to life because neither a male rabbit nor a female rabbit alone can perform reproduction [24], and how can it “undergo Darwinian evolution”? This is a more convincing example – that is, not only the incapability of reproducing, but also the incapability of performing reproduction alone – which is ordinary in the ubiquitous sexual reproduction, can cast doubt on the notion of confusing a living entity with its life form. When talking about Darwinian evolution, what we refer to is the “life form”. Indeed, if we do want to relate an entity with Darwinian evolution, we had better use the wording like “engaging into” instead of “undergoing”.