“What is life?”: Different Answers to a Simple Question in View of Synthetic Biology


Click here for a copy of the annotated bibliography on synthetic biology and the creation of life

The title of the SYBHEL Work Package 2 for which the SYBHEL-participants from the IBME (Institute of Biomedical Ethics) at the University of Zurich are responsible is: “Synthetic Biology and the Definition and Creation of life”. We enquire the impact of different definitions and conceptions of life for the assessment and understanding of synthetic biology. Synthetic biologists aim at designing and producing new life forms. This leads to discussions about what “synthetic organisms” could look like or what it means to “produce or create life”. For this Work Package, we are interested in how different scientific, philosophical or religious conceptions of life deal with these questions. Moreover, we will address how these positions assess potential medical applications of synthetic biology.

Without any doubt the aims, methodologies and techniques of synthetic biology tighten and deepen the connection between technology and living organisms. The idea is that with synthetic biology biological systems should in the future be at human disposal to an unprecedented extent. It should become possible to design and produce them such that they can fulfill human purposes to a more specific and more complete degree than in conventional biotechnology.

It is normal for the progression of a technology that control mechanisms are improved or that novel applications are developed, which can raise ethical issues. Synthetic biology (and biotechnologies in general) differs from other forms of technology with regard to the type of objects that are produced or modified. Synthetic biology deals not with traditional machines but with “biological systems”. For many people these “objects” have a particular meaning and raise different associations independently of their role in technology. This variety is illustrated by the fact that biological systems are also called “organisms” or “living beings”. These phrases are used to talk about biological systems as part of our daily environment, our natural environment and last but not least, about us as biological systems.

The different ways to name and conceive of living objects rest on different conceptions and understandings of living organisms and of life. For some philosophical or religious positions the idea of “synthetic organisms” may be problematic or raise concerns whereas for other positions it is obvious and fascinating.

In this column I want to introduce different conceptions of life and discuss interesting or controversial aspects about this phenomenon. Thereby, I pay particular attention to the questions 1) how these different understandings of “life” relate to the aims of synthetic biology and 2) whether they have an impact for the assessment of synthetic biology.

Anna Deplazes-Zemp


LATEST:  10 January, 2012



In the 1999 American science fiction drama “The Bicentennial Man” the robot Andrew played by Robin Williams is introduced into a family home to perform housekeeping. Andrew accidently starts to develop his own personality, and becomes an intelligent, creative, self-conscious, autonomous robot that desires his freedom and wishes to become a human. The film explores various issues often debated in context of artificial intelligence such as autonomy, freedom and humanity.[1] Although invented by humans to meet their desires and to satisfy them, Andrew develops an autonomous identity and wants to follow his own desires. The film raises the question what distinguishes the robot Andrew from humans and whether a robot can be autonomous and free.

In the debate on synthetic biology questions occur that are very similar to the issues raised about artificial intelligence in the film “The Bicentennial Man”: important questions are, for instance, what distinguishes synthetic organisms from non-synthetic organisms or whether artificial life synthesized by humans is a machine.[2] A machine is characterized as being determined and controlled from the outside by the purpose that humans give this machine. Thus, being a machine would imply that synthetic organisms can be used for instrumental reasons and should fulfil human purposes. However, a living organism is characterized as a self-determined system that sets and achieves its own objectives. In brief, machines are heteronomous systems whereas living organisms are autonomous systems.[3] Thus, there is a tension between the autonomy of living organisms and the external purposefulness of machines. Having this in mind, do synthetic organisms as “biological machines” lack autonomy? And what would this mean from a philosophical point of view?

Autonomy as a fundamental property of life

In their concept of autopoiesis the biologists Humberto Maturana and Francisco Varela describe the phenomena of life as autonomous, having individuality and being unities.[4] From a biological point of view, autonomy can be defined as the self-determination of organisms, i.e. the ability of an organism to act independently without being forced from outside purposes. It can be applied to different levels of life, e.g. the mental, behavioural, neurological or metabolic level. A basic form of metabolic autonomy can be formulated as the “capacity of a system to manage the flow of matter and energy through it so that it can, at the same time, regulate, modify, and control”.[5] This basic autonomy is characteristic for all internal self-constructive processes in life as well as for all processes of exchange with the environment. With regard to this, basic autonomy is a fundamental property of all living beings.

It is possible to create an artificial system which shows basic autonomy. A minimal set of three components are needed: (1) a semipermeable membrane to establish a border to the environment with the ability to interact with it at the same time, (2) a group of energy currencies like ATP as well as protons or sodium ions across the membrane for transport processes, and (3) a set of catalysts like polymers for modulating rates at which reactions take place, for setting up regulation mechanisms, and for carrying out mediated transport processes.[6] Although such a system – which can be called proto-cell – is autonomous and contains everything that is required for life, we would not call it life. In order to call it life, the long-term maintenance of the basic autonomous system should be guaranteed. Therefore, the system needs the potential to reproduce its basic functional-constitutive dynamics.

Kepa Ruiz-Mirazo, Juli Peretó and Alvaro Moreno call this potential of life to reproduce the open-end evolutionary capacity.[7] The capacity for open-end evolution needs a genetic system for the informational record of all functional components including a mechanism of translating the information. In case of life on earth the components which record information are the RNA and DNA, respectively. Translation is the process which links the informational elements and the catalytic components of the system. To sum up, a living system is an autonomous system with the capacity for evolution and long-term sustainability.[8] This is what makes autonomy a fundamental property of life.

Synthetic organisms, even at the cellular level, would show autonomy as long as they have the capacity of basic autonomy as well as the open-end evolutionary capacity, i.e. that the organisms possess some form of genetic code. Although humans synthesize such organisms for a certain purpose, the organisms cannot be called machines, as they are no heteronomous system and do not lack autonomy as described above. But if synthetic organisms are not comparable with machines, what will this mean from a philosophical point of view?

Philosophical implications of the autonomy of life

The definition of autonomy given above is of a biological and not of a philosophical kind. Even in the concept of autopoiesis established by Maturana and Varela, autonomy is not used in a philosophical sense.[9] Their definition of autonomy has to be distinguished from a philosophical understanding of autonomy. In Kantian philosophy, for instance, autonomy is a crucial moral precondition: to be autonomous means for someone to understand himself or herself as being free, i.e. to use reasons to choose one’s own actions. Thus, from a philosophical point of view, autonomy can be described as the idea that one can live one’s life according to reasons and motives which are a product of one’s own thinking and not of the manipulative forces of others. Autonomy is connected with free will and freedom. However, it needs rationality and cognition to be autonomous. Both are only ascribed to humans in Kantian philosophy. Because of autonomy, humans are bearers of moral rights. Therefore, the Kantian principle of the “non-instrumentalisation” can be applied only to human beings.[10] In Kantian terms, the autonomy of synthetic organisms would have no moral implication how to treat them, unless these organisms do not show rationality or cognition which is similar to that of humans.[11] However, it is theoretically imaginable that one day synthetic organisms might exist who show rationality or cognition in a Kantian sense. Then the principle of the “non-instrumentalisation” could be applied to them.

Unlike Immanuel Kant, Maturana and Varela do ascribe cognition to all living beings in their concept of autopoiesis. According to this concept “living systems are cognitive systems, and living as a process is a process of cognition”.[12] This process of cognition is both the result and the precondition of being autonomous. In the concept of autopoiesis, living systems are structurally coupled with their medium, i.e. they are embedded in a dynamic of changes which can be recalled as sensory-motor coupling. This dynamic of the system is what Maturana and Varela call cognition or rudimentary form of knowledge. Cognition deals with the interactions between an organism and its environment, whereas autopoiesis focuses on the internal functioning of the organism, i.e. its metabolism.[13] To put it briefly, cognition is the ability of a system to make a clear distinction between itself and the environment through own efforts. Although the term “cognition” is really misleading and should be avoided, as primitive organisms cannot show a cognition which we know from humans (in terms of reason), Maturana and Varela seem to stress that living organisms interact with their environment in a dynamic way – an ability that non-living matter cannot show. In this sense it should be understood in the following.

In the concept of autopoiesis, cognition is defined in a complete other way than in Kantian philosophy: the process of cognition in rational organisms such as humans (or great apes) is different from the process of cognition in single cells or (primitive) non-human organisms. However, cognition as the “dynamic of changes” of living systems or the “rudimentary form of knowledge” – as named by Maturana and Varela – is something that can be observed in all life-forms. This is what distinguishes life from robots, machines or artificial intelligence systems. Although such systems might one day show similar autonomy and cognition as it can be found in living systems, so far they are only to a limited degree autonomous and do not possess an open-end evolutionary capacity similar to that of organisms. Their autonomy is limited by the programs humans invented for these systems. However, synthetic organisms are different to robots, machines or artificial intelligence systems, as it is autonomy (including the open-end evolutionary capacity) which defines life and, thus, also defines them.

As we have seen, from a biological as well as from a philosophical point of view, autonomy can be defined as the characteristic of living organisms regardless of whether they are synthesised by humans or not. Two main morally relevant considerations follow from this conclusion for synthetic biology:

  • Unexpected consequences: As autonomous systems, synthetic organisms might interact with the environment in a way that has never been predicted. Therefore, unknown risks for human health or the environment should be taken into account, if synthetic organisms are released into the environment.
  • Moral value: There is broad debate in environmental ethics how to define the intrinsic value of organisms. If an intrinsic value is ascribed to the autonomy of living beings, then the autonomy of synthetic organisms has to be respected and should not be infringed. This means that synthetic organisms can only be treated in a certain way.

Concluding remarks

Life shows both the capacity of basic autonomy and the capacity of open-end evolution. Thus, it can be distinguished from machines, robots and artificial intelligence systems. Although synthetic organisms are often called “biological machines”, they are living systems due to their capacity of autonomy. Autonomy is inextricably tied up with concept of autopoiesis. The process of autopoiesis is individual for each living system as the interactions of the living system with its environment are always different. This gives each living system an individual identity.[14] In order to maintain this identity, every living system must constantly change its material composition by metabolizing nutrients from the environment. Although atoms are constantly replaced within the living system, the system remains its individual identity. Besides showing the capacity of autonomy, having such an identity is certainly what distinguishes organisms from non-living natural resources or man-made machines, which is also true for synthetic organism.

Footnotes and Literature

[1] Anderson, S. Asimov’s ‘Three Laws of Robotics’ and machine metaethics. In: Anderson, M. Anderson, S., Armen, C. (Eds.). Machine ethics: Papers from the AAAI Fall Symposium, Technical Report FS-05-06. (AAAI Press, Menlo Park, CA; 2005).

[2] Deplazes, A., M. Huppenbauer. Synthetic organisms and living machines. Positioning the products of synthetic biology at the borderline between living and non-living matter. Systems and Synthetic Biology 3, 55-63 (2009).

[3] Thompson, E. Mind in life: biology, phenomenology, and the sciences of mind. (Harvard University Press, Cambridge; 2007); q.v. page 37.

[4] Maturana, H. R., F. J. Varela. Autopoiesis and Cognition: The realization of the Living. (D. Reidel Publishing Company, Dordrecht; 1980); q.v. page 80f.

[5] Ruiz-Mirazo, K., A. Moreno. Basic Autonomy as a Fundamental Step in the Synthesis of Life. Artificial Life 10, 235-259 (2004); q.v. page 240.

[6] Ruiz-Mirazo, K., J. Peretó, A. Moreno. Universal Definition of Life: Autonomy and open-ended evolution. Origins of Life and Evolution of the Biosphere 34, 323-346 (2004); q.v. page 333.

[7] L.c., q.v. page 330.

[8] Ruiz-Mirazo, K., A. Moreno. Basic Autonomy as a Fundamental Step in the Synthesis of Life. Artificial Life 10, 235-259 (2004); q.v. page 254.

[9] Brenner, A. Living life and making life. Tymieniecka, A.-T. (ed.), Analecta Husserliana CX (Dodrecht, Heidelberg, London, New York: Springer; 2011), 91-102; q.v. page 94.

[10] Kant, I. Grundlegung zur Metaphysik der Sitten (1785). Published by: Karl Vorländer. (Philosophische Bibliothek 41). (Fleix Meiner Verlag, Hamburg; 1994).

[11] Some philosophers state that certain animals do show rationality or cognition so that their autonomy is morally relevant. Such animals would be great apes, for example. Harvard law professor and animal rights advocate Stephen M. Wise applies the theory of “practical autonomy” to such animals. This theory implies that animals should be granted legal personhood if three criteria are fulfilled: (1) species can desire; (2) species can intentionally act to fulfill his or her desires; (3) species possesses a sense of self sufficiency to allow his or her to understand, that it is he or she who wants something. For further details see: Wise, S. M. Drawing the line: Science and the Case for Animal Rights. (Perseus Publishing, Cambridge, MA; 2003); q.v. page 7, 32.

[12] Maturana, H. R., F. J. Varela. Autopoiesis and Cognition: the realization of the Living. (D. Reidel Publishing Company, Dordrecht; 1980); q.v. page 13.

[13] Bourgine, P., Stewart, J. Autopoiesis and cognition. Artificial Life 10, 327-345 (2004); q.v. page 338.

[14] Luisi, P. L. The Emergence of Life. From Chemical Origins to Synthetic Biology. (Cambridge University Press, Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo; 2006); q.v. chapter 8.


Daniel Gregorowius



November 10, 2011




Prince Charles once claimed that gene technology takes mankind “into realms that belong to God and God alone”.[1] His statement is often cited to criticize the application of biotechnology in agriculture. It implies that there is a certain order or purpose in nature which is independent from humans and set by a divine force. In this context, the notion of the “telos of nature” is often addressed in the debate on agricultural biotechnology. But what does it mean?

Telos is the Greek word for “the end of a goal-oriented process”, or simply “purpose”. It presupposes the purposiveness of living things, i.e. that they are designed to fit a certain purpose. This presupposition has been called the “teleological argument” which can be traced back to Aristotle. He stated that living things have four causes.[2] The causa materialis (material cause) addresses the substance and physical matter of an organism, whereas the causa formalis (formal cause) is the shape or form of an organism (morphology). However, the causa efficiens (efficient cause) is equivalent to the causes of motions or changes in an organism, i.e. the proximate force which is responsible for the form of an organism. Finally, the causa finalis (final cause) can be called the telos.

One might say that speaking of telos in sense of the causa finalis means that life cannot have arisen without a wise natural force that gives life its purpose. This implication is often criticized as it necessitates the existence of an intelligent creator or God. However, the concept of telos does not argue for a creator in any case. Within the debate on agricultural biotechnology there are different examples how telos can be used without arguing for a creator. Some of these examples will be given in the following part without requirement on completeness.

The concept of “telos” in the debate on agricultural biotechnology

The concept of ‘telos’ is often addressed in the debate on agricultural biotechnology to characterize the intrinsic value of living entities: it can be applied to any living entity, whether a single organism, a species, an ecosystem or nature as a whole.[3] According to Paul Taylor, each single organism is a “teleological (goal-oriented) center of life, pursuing its own good in its own unique way”.[4] This does not mean that an organism achieves its own goals intentionally or consciously, but rather that “a living thing is conceived as a unified system of organized activity, the constant tendency of which is to preserve its existence by protecting and promoting its well-being”.[5]

Concerning the intrinsic value of individual animals, Bernard Rollin goes in the same direction and argues that the animal’s well-being involves “both control of pain and suffering and allowing the animals to live their lives in a way that suits their biological natures”.[6] According to Rollin, how animals can live their life is determined by their telos which is genetically based.[7] By expressing needs or interests, animals follow their inherent species-specific and genetically based telos. However, this interpretation of telos assumes that the genetic set characteristic for a species determines the purpose of an organism.

Holmes Rolston, III also states that telos is coded in the organisms’ genetic set which is “evidently the property of the species as of the individual through which it passes”.[8] Based on this understanding, telos is characterized by relative autonomy and integrity. Rolston portrays this characterization as a loose form of teleology.[9] Following Rolston’s reasoning, to modify life technically would be problematic as it is an infringement of the organisms’ or species’ autonomy and integrity which is valuable per se. However, according to Rolston not all human interferences with nature are an infringement of autonomy or integrity.

Telos can also be assigned to nature as a whole: Eric Katz, for instance, argues that complex natural systems and biotic communities exhibit autonomy similar to species or single organisms.[10] The domination of nature by humans militates against this autonomy, as it attacks the pre-eminent value of the self-realization of nature.[11] In other words: the engineering of life would be morally wrong because it is against the autonomy of nature defined as the inherent purpose of self-realization.

In a similar way, Edith Lammerts Van Bueren and Paul Struik define telos as a form of “functional determinism”,[12] which implies that organisms – or life as a whole – accomplish a natural aim that is independent from any instrumental purpose set by humans. The telos of living entities characterizes their integrity. For instance, the integrity of life as a whole is defined by the self-regulation of all living beings who are present in the world to achieve a certain telos.[13] In this sense, integrity of life is the state of wholeness or completeness of living beings allowing them to perform all the functions that are characteristic for the members of the biotic community. In a similar way, Henk Verhoog and Jan Vorstenbosch interpreted telos as the integrity of animals.[14]-[15]

What we can learn about “telos” for the debate on synthetic biology

To summarize the various interpretations used in the debate on agricultural biotechnology, telos can be characterized as the needs or interests of a living entity, the species-specific genetic set of an organism, or as the autonomy, integrity, or self-realization and self-regulation of life. These different characterizations have in common that living entities are seen as independent identities that should be treated as an end in themselves and not merely as a means. In other words: a living entity can be seen as something that sets its own sense.[16] Having said this, telos can be interpreted in sense of the Kantian principle of the “non-instrumentalisation” of a person[17] – in this case, the “non-instrumentalisation” of all living entities.

What does this interpretation of telos mean for the “creation of life” in synthetic biology? One might say that creating life would be morally wrong, if it was solely for instrumental purposes. Thus, synthetic biology would be morally wrong as life is created in this context for human purpose only. But life is also put into existence for purely human purpose in other circumstances, for instance, when crops are grown to make flour out of them, or when people breed dogs to keep them as pets. For practical reasons, it would be problematic if any human intervention in nature that puts life into existence for purely instrumental reasons would be morally wrong. However, there is a difference between creating life by traditional breeding techniques or by genetic engineering techniques. Although this difference is important for the debate on synthetic biology, the “instrumentalisation” of life by humans might be the same in any case. Due to the concept of telos, it would be more suitable to differ between creating life for human purpose and treating it only in an instrumental way.[18] Thus, it would be worth applying the concept of telos to the treatment of life.

Treating life in a non-instrumental way means that humans should respect its autonomy, integrity, self-realization, and self-regulation. Even if organisms are designed by humans and, thus, are not independent from their purposes, they will be independent in the way they strive to live, they interact with their environment, thrive and deteriorate, regenerate, or grow and develop during their lifetime (including the influence of evolutionary forces). This is characteristic for all living entities and makes them distinct from the non-living matter.

To describe and explain the nature of living matter in this way, the concept of autopoiesis was developed by the biologists Humberto Maturana and Francisco Varela.[19] Like the concept of telos, autopoiesis also relies on the autonomy of life. This autonomy of life might be the reason for the unease that people have when they think of biotechnology or synthetic biology. Their unease is not irrational, as is often stated, it just shows that there is a clear difference between living matter and non-living matter which can be described in terms or telos or autopoiesis, respectively. Maintaining this distinction could clarify the debate on synthetic biology, because it calls for a different treatment of life and non-life, regardless of whether it is synthesized by humans or of “natural origin”.

Concluding remarks

The concept of telos can be interpreted in context of synthetic biology as the characterization of life as an independent identity that should be treated as an end in itself. Telos is closely linked to the concept of autopoiesis. Both concepts assume that life shows a form of autonomy that distinguishes it from the non-living matter. If telos is applied to the treatment of life, it means from an ethical point of view that humans should respect the autonomy of life or, in other words, the unavailability of life in treating it. Synthesizing life is not per se against the autonomy of life. Therefore, the ethical debate on synthetic biology should be more focused on how artificial life should be treated, if it is once “created” by humans.

Footnotes and Literature

[1] See: Prince Charles. Seeds of disaster. The Daily Telegraph, 8th June 1998

[2] Aristotle. Aristotle in 23 Volumes; Vols. 17 and 18. Translated by Hugh Tredennick. (Harvard University Press, Cambridge, MA; William Heinemann Ltd., London; 1933, 1989); q.v. Aristot. Met. 5.1013a.

[3] See: Lammerts van Bueren, E. T., P. Struik. Integrity and Rights of Plants: Ethical Notions in Organic Plant Breeding and Propagation. Journal of Agricultural and Environmental Ethics 18, 479-493 (2005).

[4] Taylor, P. W. Respect for Nature. A Theory of Environmental Ethics. 25th Anniversary Edition. (Princeton University Press, Princeton, NJ; 2011); q.v. page 45.

[5] L.c., q.v. page 45.

[6] Rollin, B. The Frankenstein Syndrome. Ethical and Social Issues in the Genetic Engineering of Animals. (Cambridge University Press, Cambridge, MA; 1995); q.v. page 157.

[7] L.c., q.v. page 159.

[8] Rolston III, H. Genes, Genesis and God. (Cambridge University Press, Cambridge; 1999); q.v. page 42.

[9] L.c., q.v. page 367.

[10] Katz, E. Artefacts and Functions: A Note on the Value of Nature. Environmental Value 2, 223-232 (1993); q.v. page 230.

[11] L.c., q.v. page 230.

[12] Lammerts van Bueren, E. T., P. Struik. Integrity and Rights of Plants: Ethical Notions in Organic Plant Breeding and Propagation. Journal of Agricultural and Environmental Ethics 18, 479-493 (2005), q.v. page 482.

[13] L.c., q.v. page 482.

[14] See: Verhoog, H. The concept of intrinsic value and transgenic animals. Journal of Agricultural and Environmental Ethics 5, 147-160 (1992)

[15] See: Vorstenbosch, J. The concept of integrity: its significance for the ethical discussion on biotechnology and animals. Livestock Production Science 36, 109-112 (1993).

[16] Brenner, A. Leben: eine philosophische Untersuchung. (Bundesamt für Bauten und Logistik BBL, Bern; 2007); q.v. page 112.

[17] See: Kant, I. Grundlegung zur Metaphysik der Sitten (1785). Published by: Karl Vorländer. (Philosophische Bibliothek 41). (Fleix Meiner Verlag, Hamburg; 1994).

[18] Link, H.-J. In-Depth analysis of outstanding philosophical issues, in: Synth-Ethics Consortium (ed.). Synth-Ethics: Identification of ethical issues and analysis of public discourse. FP7 Report WP1 (deliverable 1), 40-49 (2010); q.v. page 43.

[19] See: Humberto, R. M., F. J. Varela. Autopoiesis and Cognition: The realization of the Living. (D. Reidel Publishing Company, Dordrecht; 1980).


Daniel Gregorowius



September 22, 2011


One might expect that biologists, as experts on the “science of life”, should be able to provide an answer to the question “What is life”. However, biologists normally do not deal with this general question but rather focus on specific aspects of and processes in  living organisms. When biologists are confronted with the question “What is life”, they often respond with a list of biological attributes that they associate with life or living organisms. For instance Judith G. and Donald Voet, the authors of a standard biochemistry textbook, refer to the three criteria proposed by Norman Horowitz: replication, catalysis and mutability.1 Scientists in protocell synthetic biology, who aim at producing artificial cells from nonliving material, have suggested the three characteristics: metabolism, genes and container.2

Many other attributes have been suggested as signs or hallmarks of life, normally they are assembled in lists.(for example 3-6) The following points can often be found amongst these hallmarks:

Genome: Most, if not all of these lists include a point that refers to the fact that living organisms carry their own “programme” in the form of a genome.

Faultiness: Sometimes, a separate point is dedicated to the fact that replication of the genetic programme is not perfect and that mutations can occur. This faultiness leads to genetic variation.

Evolution: Living organisms form populations that can evolve. This means that evolutionary mechanisms such as genetic variation, genetic drift and natural selection lead to varieties within species and the production of new species.

Metabolism: All living organisms have a metabolism by which they convert energy and matter that they take up from the environment. This metabolism consists of a highly complex set of biochemical pathways, which are coordinated by sophisticated mechanisms.

Homeostasis in an open system: Living organisms maintain an internal equilibrium in spite of the changing external environment. This is possible thanks to elaborated regulation mechanisms although the organism is not a closed system but an open system with constant exchange of material and energy with the environment.

Interaction with the environment: The previous point is one example of how living organisms constantly interact with the environment by reacting to changing conditions. Other examples include interaction with other members of the species or organisms of other species.

Control and Regulation: Several amongst the previously mentioned points depend on a very sophisticated and flexible control and regulation system that is characteristic for living organisms. A special aspect about these control and regulation mechanisms is that they are exercised by the controlled entities themselves. This gives living organisms a certain autonomy. Feedback mechanisms play an important role here.

One entity: Living organisms are confined individual entities that cannot be divided without losing their essential properties. Moreover, they are surrounded by a surface and they perform certain activities as an entity.

Isolation: In living systems many different reactions go on at the same time. Although certain interactions between these reactions are necessary, it is important that they do not interfere with each other where this is unrequested. This isolation is implemented, for instance, by means of compartimentalization, organisms consist of different compartments with specialized functions. Moreover, enzymes display specificities, which ensures that they react only with certain molecules but not with others.

Growth, Replication: Many lists of signs of life refer to the fact that living organisms grow at some point in their development and that they are able of replication at the levels of molecules, cells and complete organisms. The replication of certain structures in the organism also allows for regeneration of degraded structures.

Chemical properties: Some lists include a point in which certain chemical properties are described as characteristic for living organisms, for instance the occurrence of nucleic acids, proteins or lipids.

These points – or a selection of them – appear on different lists under varying names. Sometimes certain attributes are combined into one, or others are split into several points. The selection of attributes and the names chosen for these signs depend on the focus of the author.3-6 The two examples of three signs of life mentioned at the outset illustrate how the selection and naming of hallmarks can indicate a specific emphasis. The Horowitz-list has been published in a paper on the origin of life. In this context, scientists emphasised the importance of evolution as the driving force behind the diversity of life as we know it today. Therefore, evolution is considered with the two hallmarks: replication and mutability. The list that was set up by researchers interested in the production of artificial cells does not directly refer to evolution. In this context, the various components of a living organism are of high interest, because researchers want to know from what components they can assemble an organism. Therefore, genome and container appear as attributes on the list. “Catalysis” in the Horowitz-list and “metabolism” in the PMC-model both refer to the biochemistry of living organisms. . The first term, catalysis, refers to something that can be measured whereas “metabolism” refers to what synthetic biologists want to design. These two short lists show how similar notions can be expressed differently and explain why lists of signs of life differ.

What can lists of signs of life tell us?

In some contexts, these signs of life are understood as criteria, which should enable us to recognize the occurrence of life. This would be useful for astrobiologists, who want to identify life in outer space, or for protocell synthetic biologists to decide at what point an artificial construct could be called alive. Lists used in this sense, describe life, but they cannot explain what life is and why the different signs are characteristic for the occurrence of life. Although the different attributes are tightly connected and dependent on each other, the list does not point out how they belong together. For this reason, some authors hold that such lists cannot provide a satisfying answer to the question “What is life”.78

Marc Lange suggested that the relation between life and signs of life is not that the latter define the former.9 Rather, the significance of the signs of life is that they bear a special relation to life.10 According to Lange, these attributes are signs of life if the fact that an entity displays them can be explained by its being alive. For instance, a plant grows because it is alive whereas a crystal grows for other reasons. The discussion of signs of life thus only makes sense in connection with a previous idea of what life is.

The significance of the various signs of life

Lange’s analysis indicates that it is not clear, whether the attributes on these lists should be understood as criteria for life, as indicators for life or as expressions of life. If they are understood as criteria the question is, whether each criterion is necessary or whether the occurrence of singular criteria is considered to be necessary to ascribe life. If one decides for the second option, one may wonder whether all criteria have the same weight or whether the occurrence of certain criteria is more decisive than that of others.

Moreover, it is not only unclear what attributes are necessary to ascribe life, but also what is considered to be sufficient for this purpose. Is a single criterion sufficient to ascribe life, are the criteria sufficient as a set or is there no claim for sufficiency at all?

Often the not explicitly mentioned idea behind such lists seems to be that together the criteria are sufficient to ascribe life and that at least some of the singular criteria are necessary.

Entities that we intuitively would consider alive, but that do not show all signs of life (for instances mules or other organisms that cannot reproduce) or entities that do fulfil certain criteria but do not seem to be alive (growing crystals) have been brought forward to question the usefulness of lists of signs of life.78 Furthermore, some biological entities such as viruses, frozen bacteria, prions or endosymbionts, which fulfil some but not all criteria of life, are discussed as borderline cases between life and non-life.711 As long as we do not have a clear weighing of the significance of the different criteria it is difficult to decide whether such entities should be considered alive.

Application to synthetic biology

As mentioned at the outset some protocell synthetic biologists have suggested a list of criteria of life. This indicates that the description of life based on a list may be useful for researchers who aim at producing artificial cells. Necessary criteria on the list provide the conditions that the cell needs to fulfil for that it can be called alive. In other branches of SB the question of whether or not their product is alive is not so important. For these scientists it may be more interesting to ask which criteria decide whether an organism should be called synthetic. Is, for instance, a bacterium with a synthetic genome synthetic or what about a bacterium with a human designed metabolism? For synthetic biologists the different criteria of life provide a set of possible starting-points to redesign life and tools to modify living organisms for useful purposes.12

Footnotes and Literature

1. Voet, D. & Voet, J.G. Biochemistry, Edn. 2. (Wiley 1995).

2. Rasmussen, S. et al. in Protocells, Bridging Nonliving and Living Matter. (eds. S. Rasmussen et al.) xiii-xxi (The MIT Press, Cambridge, MA London; 2009).

3. Deamer, D. Special collection of essays: what is life? Introduction. Astrobiology 10, 1001-1002 (2010).

4. Ganti, T. The Principles of Life. (Oxford University Press, Oxford; 2003).

5. Koshland, D.E., Jr. Special essay. The seven pillars of life. Science 295, 2215-2216 (2002).

6. Mayr, E. This is Biology: The Science of the Living World. (Harward University Press, Cambridge, MA; 1997).

7. Bedau, M.A. An Aristotelian account of minimal chemical life. Astrobiology 10, 1011-1020 (2010).

8. Thompson, M. (Harvard University Press, 2008).

9. Lange, M. Life, “Artificial Life,” and Scientific Explanation. Philosophy of Science 63, 225-244 (1996).

10. Lange uses the term “vitality” by which he means the “being alive” of an entity, this is not to be confused with vitalism.

11. Dupré, J. & O’Malley, M.A. Varieties Of Living Things: Life At The Intersection Of Lineage And Metabolism. Philosophy & Theory in  Biology 1 (2009).

12. Deplazes-Zemp, A. The Conception of Life in Synthetic Biology. Science and engineering ethics (2011).


Anna Deplazes-Zemp



April 21, 2011


In this entry, I would like to discuss some general difficulties with the term “life”. One difficulty arises from the fact that it is not clear what the term “life” actually denominates. Another problem is that the term ”life” has different meanings depending on the context. This is illustrated by the different meanings of the two Greek words for “life”: “zoë” and “bios”.

What does the term “life” denominate?

What does a speaker describe when she/he uses the term “life”: a phenomenon, a property, an activity, a process, a form of existence, a set of conditions, an entity or something else again? Some authors explicitly mention one of these options, but most of the time the reference of the term “life” remains elusive.

The fact that this term can be applied at the individual as well as at the collective level adds to its fuzziness. If people talk for instance about metabolism or subjectivity as characteristics for life, they mean life of a singular individual. However, if evolution or interaction between organisms is mentioned as what characterizes life, then “life” is understood as something that occurs in a group of individual organisms.

In many cases, when characteristics for life are discussed in context of individual organisms, it would be clearer to speak of characteristic of “living organisms” instead of “life”. This is particularly true for discussions about synthetic biology. When authors write about the creation or production of life they usually refer to the production of a specific cell or microorganism. If life is understood as a phenomenon, a property or an activity it is questionable whether this is something that can be “produced”.

Zoë – Bios

Philosophers such as Hannah Arendt or Giorgio Agamben refer to two different terms for “life” in ancient Greek in order to point to another ambiguity in the English term. This is the distinction between zoë and bios as it is lead back to Aristotle. “Zoë” stands for “biological life” whereas “bios” refers to a personal type of life. These words form the roots for the words “zoology” as the biological life of animals and “biography” as the personal type of life of an individual person.1 Thomasine Kushner distinguishes between the two terms by translating zoë as “being alive” (in the biological sense) and bios as “having a life” (in the biographic sense). 2

Depending on whether we think of life in the zoë-sense or the bios-sense we associate different connotations with it. This is particularly true for normative aspects. Many people would agree that life as bios has value and needs to be morally considered whereas this may be more controversial for life as zoë.

Another interesting difference concerns the contrary of the adjective “alive”. Again this difference could be related to that between zoë and bios. The contrary of “alive” in the sense of zoë in a biological living system would be “lifeless” or “inanimate”. The contrary of alive in the biographical sense would be “dead”.

The awareness of these ambiguities of the term “life” may be useful for the comparison between different conceptions of life and it may help to explain certain differences.

Footnotes and Literature

1 Note that in the word “biology” the root “bios” is used with a different meaning.

2 Kushner, T. Having a life versus being alive. J Med Ethics 10, 5-8 (1984).


Anna Deplazes-Zemp





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