To date, no definition of life has been unequivocally accepted by the scientific community. In frustration, some authors advocate alternatives to standard definitions. These include using a list of characteristic features, focusing on life’s effects, or categorizing biospheres rather than life itself; treating life as a fuzzy category, a process or a cluster of contingent properties; or advocating a ‘wait-and-see’ approach until other examples of life are created or discovered. But these skeptical, operational, and pluralistic approaches have intensified the debate, rather than settled it. Given the failure of even these approaches, we advocate a new strategy. In this paper, we reverse the usual line of reasoning and argue that the “life problem” arises from thinking incorrectly about the nature of life. Scientists most often conceptualize life as a class or kind, with earthly life as a single instance of it. Instead, we advocate thinking about Earth’s Life (with a capital ‘L’) as an individual, in the way that species are now thought to be. In this view, Life is a monophyletic clade that originated with a last universal common ancestor, and includes all its descendants. We can continue to use the category ‘life’ (lower case ‘l’) pragmatically to refer to similarities between various phenomena and Life. But the relevant similarities are a matter of interest and preference, not a matter of fact. The search for other life in the Universe, then, is merely a search for entities that resemble parts of Life in whatever sense astrobiologists find most appealing (e.g. metabolism, evolution, information, etc.). This does not mean that the search for evolved complexity elsewhere in the universe or its creation in the lab are futile endeavors, but that debates over whether they count as ‘life’ are. Ironically, finally abandoning the concept ‘life’ may make our searches for evolved complexity more fruitful. We explain why.
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The distinction between classes and categories is inconsistent across philosophical subdisciplines. We use them interchangeably throughout.
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We thank the Natural Sciences and Engineering Research Council of Canada (Grant No. GLDSU/447989) for support, as well as the audiences in POBAM 2016, Duke University, University of Washington, Bryn Mawr College, University of Nevada, Reno, Dalhousie University, SoCIA 2018, Austin Booth, Letitia Meynell, Tyler D.P. Brunet, the editors, and several anonymous reviewers for valuable comments that helped improve this paper.
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Mariscal, C., Doolittle, W.F. Life and life only: a radical alternative to life definitionism. Synthese 197, 2975–2989 (2020). https://doi.org/10.1007/s11229-018-1852-2
- Definitions of life
- Philosophy of astrobiology
- Origin of life
- Theory of life