Despite numerous and increasing attempts to define what life is, there is no consensus on necessary and sufficient conditions for life. Accordingly, some scholars have questioned the value of definitions of life and encouraged scientists and philosophers alike to discard the project. As an alternative to this pessimistic conclusion, we argue that critically rethinking the nature and uses of definitions can provide new insights into the epistemic roles of definitions of life for different research practices. This paper examines the possible contributions of definitions of life in scientific domains where such definitions are used most (e.g., Synthetic Biology, Origins of Life, Alife, and Astrobiology). Rather than as classificatory tools for demarcation of natural kinds, we highlight the pragmatic utility of what we call operational definitions that serve as theoretical and epistemic tools in scientific practice. In particular, we examine contexts where definitions integrate criteria for life into theoretical models that involve or enable observable operations. We show how these definitions of life play important roles in influencing research agendas and evaluating results, and we argue that to discard the project of defining life is neither sufficiently motivated, nor possible without dismissing important theoretical and practical research.
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The way we use the term here should also not be confused with the broader notion of operationalism, i.e., the view of some logical positivists that the meaning of a term bears solely on the methods for its empirical measurement.
In the same paper Machery (2012) also proposes an argument against definitions of life in folk psychology. We will not address it here, as we are interested in their use in science.
The famous example considers two planets which are exactly the same except that the substance they call water, and which exhibits the same sensible properties, has a different composition on each planet. Before such chemical composition is discovered, two identical individuals with identical mental states, living on the two planets, would both call the substance ‘water’. They have the same concept of what water is and would think water is the same on both planets. But once scientists discover the different chemical composition of the two substances on the respective planets, they show that using the same term is wrong: “It follows that the extension of the term ‘water’ is not fully determined by concepts in the mind” (Cleland 2012, p. 134). Putnam’s example has been strongly criticised, as admitted by Cleland herself. Yet, she claims, it exposes an uncertainty in the relationship between concepts and natural kinds that is sufficient to undermine definitional approaches.
According to Brigandt, “[f]or any kind, the philosophically relevant question is an epistemic issue: how scientifically important is the grouping of an object into a kind, i.e., what generalizations and explanations can the kind figure in, and how important are they?” (Brigandt 2011). See Diéguez (2013) for a position that combines ontological and practical claims. See Amilburu (2015) for a recent detailed classification and discussion of different approaches to natural kinds.
The analysis is based on published papers as well as personal interactions between one of the authors and Luisi and his team for more than a decade.
A micelle is a spherical aggregate of lipid molecules characterised by a hydrophilic polar head directed towards the solvent and a hydrophobic tail directed towards the interior. A lipid vesicle is a structure characterised by a fluid core enclosed by a lipid bilayer. See Stano and Luisi (2016), for a recent historical review of the main research lines developed by Luisi’s research teams in Zurich and Rome.
“[The autopoietic organisation] (...) is a network of production processes (transformation and destruction) of components which produces the components which: (1) Through their interactions and transformations, permanently regenerate and realize the network of processes (relations) which produces the components; and (2) constitute a concrete unity in space, within which they (the components) exist by specifying the topological domain of its realization in that network.” (Maturana and Varela 1973, 1980, p. 79).
Varela et al. (1974) proposed the definition of an autopoietic system together with a computational model of the generation and maintenance of a compartment. Thus, early research on Artificial Life was directly related to a specific definition. The relationship between the metabolism and the compartment had been stressed in the same years by Gánti (1979) through his model of the chemoton, thought of as a possible realisation of a definition of a minimal living system. Some of the first (but ultimately unsuccessful) experimental attempts to synthesise an autopoietic system were performed by Gloria Guiloff, a graduate student in Maturana’s laboratory at the Universidad de Chile (see Guiloff 1981).
Oleate vesiscles are spherical bilayer structures that host an aqueous core, and are composed of simple long chain fatty acids (such as oleic acid) that are ionised to form hydrogen bonds.
Today, more sophisticated forms of such experiments are common both in wetware and software domains, but at the time they were unusual (see Luisi 2015). For a comprehensive review of this approach in current systems chemistry see Ruiz-Mirazo et al. (2014). For recent examples of wetware and software applications see Murillo-Sanchez et al. (2016) and Agmon et al. (2016), respectively.
See Oberholzer et al. (1995) for a preliminary realisation of this idea in oleate vesicles by Luisi’s team at ETH-Zurich. This work already shows a flexible attitude towards combining different definitions: “by combining the RNA replication with the principles of autopoiesis, we obtained a bridge between the two more accepted views on the theory of minimal life, the one based on the “RNA-world” and the other based on the cellular autopoietic view” (See Oberholzer et al. 1995, pp. 255–256).
Szostak has recently criticised the effort of defining life on the grounds that the origins of life concern transitions, but definitions of life do not tell us how these transitions took place (Szostak 2012; see also Trifonov 2012). Yet, although definitions do not tell what happened, they can guide the scientist in selecting which features to examine: they are not answers, but tools. Szostak himself defines life in terms of Darwinian evolution, considered as the “unifying characteristic of all Biology” (Szostak 2012, p. 599). Accordingly, he focuses primarily on realising in the laboratory those transitions that give rise to conditions for evolution, such as the combination of template replication and protocell division (see Mansy et al. 2008; Adamala and Szostak 2013).
Liposomes are vesicles composed of phospholipids, the lipids that compose current cell membranes. They are more stable but less permeable than oleate vesicles.
The role of definitions of life related to ethics has ramifications that extend to environmental ethics and medicine (Machery 2012). The role of definitions of life in ethics is beyond the scope of this paper, which is focused on the role played by definitions in the frontier disciplines aforementioned.
This is the case, for example, of those approaches which include Darwinian evolution as a crucial property to define life.
Or are identified with theoretical models of minimal living systems (see Letelier et al. 2011).
See for example Forterre (2010).
Protocells are coherent unities (spherical collections of lipids) proposed as the infrastructures for the origins of life. See Shirt-Ediss (2016) for a thorough analysis of the protocells approach to study the origins of life.
An alternative proposal advanced by Cronin et al. (2006) has been to design and implement Turing tests for lifeness, to have real cells evaluate artificial ones. Yet, the value of the test is only limited to life–like interactions.
For a discussion of the role of definitions of life in Artificial Life, see for example Umerez (1995).
Consider for instance the NASA effort to formulate a definition to help decide which experiment to realise to detect life on Mars: “Life is a self-sustained chemical system capable of undergoing Darwinian evolution” (discussed for example in Luisi 1998). For a criticism of this enterprise, see Cleland (2012).
See Ruiz-Mirazo et al. (2014) for a review of the emerging field of Prebiotic Systems Chemistry and of the role played in it by definitions of life.
The reason we emphasize necessary, rather than sufficient conditions, is that these are more pertinent tools in the scientific practices we examine here. The targets are simple life–like, prebiotic or minimal living systems, that is, systems that do not exhibit all the features of life, or just the minimal ones. Accordingly, the focus of research is on individual, or sets of, necessary conditions for life, and on their emergence or precursors in the prebiotic world.
We find that this use of definitions is better reflected by the term ‘satisficing’ rather than ‘sufficient’. The use of the term ‘satisfice’, a mix of ‘satisfy’ and ‘suffice’, has been introduced by Simon (1956) to denote a heuristic strategy according to which a decision is made in real life when it satisfies the minimum requirements necessary to achieve a certain goal (see also Gigerenzer and Goldstein 1996). It better fits our view of definitions of life, because the necessary conditions included in a definition reflect pragmatic choices that are dependent on practical and theoretical purposes. Moreover, this choice has a limited validity in time, insofar as definitions are refined in response to criticism, empirical results and new issues to be addressed.
To make it clearer, definitions whose central properties and phenomena that are not in principle or practically possible to study in the laboratory or in simulations (e.g. entelechies or unspecified dispositions) do not satisfy the operational criteria.
This is not necessary the case for all definitions. “Something is X if and only if it is red and square” does not raise problems of integration as long as ’red’ and ’square’ are independent properties. We thank an anonymous reviewer for pointing out the need to make this point more explicit.
The criteria change slightly in different publications of Gánti’s work.
The characterisation of the template subsystem as a ’regulatory’ mechanism is controversial, and it has been criticised by Bich et al. (2016).
In the operational framework proposed here, the lack of consensus does not derive from a disagreement on how to demarcate life as a natural kind. Rather, it is related to the evaluation of different research programs (or subprograms) and modelling frameworks underlying definitions, i.e., it is a lack on agreement on which are the most relevant theoretical and practical problems to be solved and questions to be asked, and how to best address them. Disagreements on definitions are in this sense not different from scientific disagreements on the best model or modelling framework for solving scientific puzzles.
A functional perspective, open to multiple realisability in the molecular domain, can be generalised to other possible forms of life, as it is not univocally committed to the exact biochemical composition of life as we know it, that is: DNA, RNA and proteins made with the specific subset amino acids of known life, the same genetic code, etc.
Strong and operational definitions are not the only possible kinds of definitions of life. Intermediate positions between these two are also possible, for example combining instrumental claims with more moderate ontological ones (an example is Ruiz-Mirazo et al. 2004), which nevertheless would require a philosophical justification against Machery’s and Cleland’s criticisms.
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The authors thank Carol Cleland for the challenging and stimulating discussion on the prospects and limitations of defining life. We also thank the other fellows of the Center for Philosophy of Science at the University of Pittsburgh during the spring term 2016: Agnes Bolinska, Andrew Inkpen, Nancy Nersessian, Mael Pegny, Mike Stuart, Matthias Unterhuber, and the Director John Norton, for the very valuable feedback. We acknowledge William Bechtel and Derek Skillings for their careful reading and useful comments on a previous version of this paper, and Alba Amilburu, Ben Shirt-Ediss, Kepa Ruiz-Mirazo, and Pasquale Stano for bibliographical suggestions. Leonardo Bich was supported by grants from the CONICYT, Chile (FONDECYT Regular 1150052), the Basque Government (IT 590-13) and Ministerio de Economía y Competitividad, Spain (FFI2014-52173-P), and a Visiting Fellowship from the Center for Philosophy of Science of the University of Pittsburgh. Revisions were done during Leonardo Bich’s postdoctoral fellowship funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme—Grant agreement no 637647 – IDEM.
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Bich, L., Green, S. Is defining life pointless? Operational definitions at the frontiers of biology. Synthese 195, 3919–3946 (2018). https://doi.org/10.1007/s11229-017-1397-9
- Definitions of life
- Origins of life
- Artificial life
- Synthetic biology
- Philosophy of science in practice