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José María Díaz Nafría
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Robin Süskind (10/01/2021, within the course "A Journey through Philosophy" facilitated by J.M.Díaz at HM)

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Abstract: The following contribution gives insight in how life was defined in the past and present. First giving some naive estimation from philosophers like Aristoteles and Democritus. Then going over some more modern definitions and their flaws, and then finally over the most up to date and accurate ones. There should also be a disclaimer that there still is no definition with which every field of science and the community is completely happy, and all the following definitions are extremely dependent on the subject they were created for and should be taken with a grain of salt. After that there is a segment, with the topic of: where we can find life and when it first occured.

1.     Intuition

For many years, philosophers had completely different views on nature and therefore life. Their hypotheses nowadays seem ridiculous but must be taken to their time, respectively. To get to a good idea of what is alive and what is considered to be dead or lifeless, one should question himself what he would consider to be alive by the first look and his gut feeling. In a poll by “The Science Asylum”¹, 2020 with 3681 votes, a good insight in the general direction was seen. There, 93% of people felt that humans are alive, 84,7% for Bacteria, 53,8% for Biological Viruses, 9,5% for computer viruses respectively and 12,2% thought that fire is alive. As seen here, it is not very easy to find a good definitiondirectly. While many of the participants agreed in the same direction, many had different views in this categorization. In a biological sense, only Bacteria and Humans from this list are alive.

2.     Proposed definitions which are nowadays not considered anymore

One of the first proposed theories of life is materialism. It states that everything is a mere complex combination of matter. Greek philosopher Empedocles (495 B.C.) argued that everything is made out of the four eternal "elements": air, water, fire and earth². Everything that exists is a combination and rearrangement of those four elements. All different kinds of life forms therefore also have to be a combination made from rearrangements of those four. This theory has now mostly been disproven. But part of this theory lives still on in the form of that we are now sure that everything is made out of a certain arrangement of elements, but the elements themselves have changed. For example: the theory states that water is one of the four eternal elements but we now know for certain, that water can be arranged by using two hydrogen atoms with one oxygen. This theory was later expanded by philosopher Democritus` (460 B.C.). As many other writers, he tried to explain what makes matter to a living thing. He stated that there has to be more to life than a combination of elements that by themselves are not considered alive. So he added to the Theory a new way of dividing between the living and the not. In summary his approach to this problem was that the essential characteristic of life is having a soul (psyche). Like everything else in the theory, is the soul made out of atoms. But he didn`t consider them to be like the rest, but fiery because he saw a connection between heat and life³. Aristoteles (384 B.C.) introduced the idea of “Spontaneous generation” which states that life can not only emerge from already living things but also from non-living things like dust, garbage or even mud.⁴ This work of Aristotle was the combination and enlargement of many prior natural philosophers and already established various theories of the appearance of organisms from seemingly out of nowhere. The theorie was held disputet for more than two millennia. It was later decisively disproven by the experiments of Lous Pasteur in 1859. In contrast to this stands Vitalism. Proposed by Georg Ernst Stahl (1659) it states that organic material can only emerge from living things. Prompting that it is impossible for life to emerge from non living things and that there is a strict difference between the living and the non living. This not only applied for living things themselves, but also for products resulting from life. This theory also was later disproven, by Friedrich Wöhler in 1828 when he was able to prepare urea (the main compound in the urin of mammals) from inorganic educts. in Vitalism, this experiment should have been impossible due to the lack of living organisms.  Because both theories were disproven, they are nowadays not considered anymore.

3.     Comparing more modern definitions with some examples

For reference, human cells and erythrocytes are considered to be alive, viruses and fire are considered to be not alive. In the following these will be used to see the effects of the different views. In biology there are seven criteria which are used to distinguish between alive and not alive. These are: Does the system conduct homeostasis, maintain organization, pursue metabolism, grow, adapt, response to stimuli and reproduce. By comparing these factors with the four examples we can already see the flaws we get by quantifying life. Human cells check all these boxes. Viruses lack the ability for a response to stimuli, own reproduction, metabolism and growth. Fire is able to conduct metabolism, growth, adapt to different diets and reproduction, but is not able to response to stimuli and maintain an organized form and therefore homeostase. Due to the fact that fire is just a accumulation of Plasma. Often it is so hot that the Molecules inside have such a huge amount of kinetic energy that homeostasis is impossible to accomplish by its natural form. It is even able to behave in different ways, depending on its surrounding. On high temperatures, pyrolyzed gases will form waves and pulsate from the ceiling, like waves in a rough sea. It will adjust its size on the concentration of educts for its reaction. And it will try to survive sudden death by suffocation by keeping a closed system on high temperature without flames, like bacteria use spores to survive harsh conditions and then incubate the whole system in a few seconds when the conditions are right again. This is why fire is often called “the enemy” by firefighters implementing that it is more than just not alive and fighting fires is often handled like a hunt of an animal. Erythrocytes, considered to be alive, don`t have their own genetic information and are therefore not able to reproduce themselves by cell division. The definition adapted by NASA “A self-sustaining chemical system capable of Darwinian evolution”, would kick out fire, because it’s not able to conduct evolution due to the fact that it has no genetic code to be altered. Erythrocytes would check this test nevertheless, because before they are differentiated they have their own DNA and only loose it in this step which is not blocking them from evolving their previous form and therefore their final form. Viruses would be discarded because they are not a system undergoing chemical reactions. They don`t have a metabolism and use their host to do the necessary work for replicating themself.

4.     The Aspect of maintaining order

As seen before in the biological definition, order and homeostasis are two very important variables to be considered. Schrödinger, 1944: “An organism`s astonishing gift of concentrating a stream of order on itself and thus escaping the decay into atomic chaos”. This implies that Schrödinger laid much weight on the Aspect of controlling order (formally known as Entropy in reference as in its micro state sense, in contrast to its macro state sense), which in his eyes differentiates between life and “Atomic chaos”. What he meant by this is that all molecules will slowly decay and destroy themselves. But living things have the ability to synthetise new ones and are then able to compensate for the decay. Lehninger ,1993 in Principles of Biochemistry, 2nd Edition: “Living organisms preserve their internal order by taking from their surroundings free Energy (also known as Gibbs Energy) in the form of nutritions or sunlight and returning to their surroundings an equal amount of energy as heat and Entropy”. This implies that living organisms have the ability to "fight" against the decay by using the energy in their surroundings, but they produce Entropy in the process and have to release it.This is also stated in the classical physical definition of life by Luttermoser: “Living beings are thermodynamic systems with an organized (order) molecular structure that can reproduce itself”.¹⁰ These definitions imply a very high contribution of entropy to be considered. Here should be stated that Entropy can be created but not destroyed. So that living organisms have to be able to dispose of it sooner or later.

5.     Entropy as the key

In thermodynamics applies the definition: “Life is an open system which makes use of gradients in its surroundings to create imperfect copies of itself”¹⁰. This definition is relatively vague and many things can fall into this category which we usually wouldn`t consider to be alive. For example it could be argued that fire matches this definition. Boltzmann made a more charging statement, (1974): “The general struggle for existence of animate beings is not a struggle for raw materials, these for organisms are air water & soil, all abundantly available, nor for energy which exists in plenty in the sun and any hot body in the form of heat, but rather a struggle for entropy, which becomes available through the transition of energy from the hot sun to the cold earth”¹¹. Further implying that entropy is the key for dividing the not living from the living, by suggesting that living organisms not only have to be able to use the energy and materials in its surroundings but also be able to control the entropy. Lovelock: “I`d look for an entropy reduction, since this must be a general characteristic of life”.¹² This entropy, called irreverversible entropie is created by any thermodynamically process like synthetising new molecules or the metabolism. It can`t be destroyed, only produced and therefore has to be discartet into the surrounding. But entropy also describes the amount and spread of molecules in a system. This can be controlled but will not happen by itself. So if a system if found organising itself and using bigger molecules to have less, than many smaller ones has to be alive by this definition. Under these rules, many other systems which are not considered alive can now be excluded. For example fire would now not match the criteria anymore, because it is not able to controll the entropie in its system and would also be excluded.

6.     Combining the learnings

The most obvious trait life must have, is being able to convert energy from a concentrated form into a usable, but still being able to have full control of entropy and the order of the system. Also the system has to be able to produce an imperfect copy of itself, in at least one stage of its life cycle. Homeostasis, growth and response to stimuli are not decisive for the definition but crucial for survival. Being able to have control of the entropy in the system consists for example of being able to produce starch or fat to combat having many small molecules like sugars or ATP and the diffusion problems that come with it. Being able to produce an imperfect copy of itself has the benefits of reproduction and therefore a higher chance for the survival of the tribe, and every copy is different, so that the perfect one survives. This is also known as Darwinian evolution. Homeostasis makes things a lot easier for the organism and growth is needed for reproduction. Response to stimuli is also handy but on a planet with energy in excess and no predators, it would be rather useless. So that the requirements can be broken down to: Energy conversion, Entropy regulation and reproduction. With this definition, human cells would still check the requirements, as well as erythrocytes. Viruses wouldn`t make it because of their lack of metabolism and self reproduction. Their whole life is based on infecting other organisms so that if all organisms would vanish, viruses would succumb to the increasing entropy and decay before being able to reproduce. Fire also wouldn`t make it. For it is not able to handle its entropy production and metabolism in a sustainable manner. In the end there has to be said that there is still not a perfect definition and we will still find many cases which lay on the border and can`t satisfy both options.

7.    Emergences

 In comparison to the 13.8 billion years old age of the Universe, the Earth is with it`s 4.54 billion years relatively young¹³. The oldest evidences of life imply that life could be as old as 3.5 billion years¹⁴, with the first water around 3.8 to 4.3 billion years ago¹. Water is considered to be the most crucial factor for the existence of life, beacuse many processes need it to be able to take place. Also these processes need a fluid in which molecules are able to diffuse about. For this, water is perfect. As today, the total umber of species is estimated to be around 8million to 100 million to 1 trillion¹⁶. From these, only 1.2 to 2 million are categorized¹⁶. The total Number of catalogued and total species is uncertain. Although the origin of life is uncertain, the most accepted theories are based on the Miller-Urey expermient and the work of Sidney Fox. These show, that the conditions on ancient earth favored the chemical synthesis of amino acids and other organic materials from inorganic educts. In spite of the complexity of RNA, it was possible to create this molecule by using more simple precursors of it¹.  Despite of it being more difficult to create it than other organic molecules like lipids. This is due to that the predicted precursors to RNA are very stable and react very slowly under normal conditions. The complexity of all the different life forms are a result of the combination and dynamic interplay between genetic variation, metabolic capability, environmental challenges and symbiosis¹. In the beginning, most of biological organisms were in the form of microorganisms. Their by-products changed the whole ecosphere of earth over a large time scale. For example, the release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this posed novel evolutionary challenges, and ultimately resulted in the formation of Earth's major animal and plant species. This interplay between organisms and their environment is an inherent feature of living systems¹. This interplay and shift of concentration of specific molecules can nowadays be seen in the form of the man made ongoing climate change, which will proceed to challenge organisms of all kind on earth, due to the warming of earth because of the rising concentration of greenhouse gases in the Atmosphere.

8. Occurrences

Organisms can not only be found on land and water, but also in the air. The whole sum of all ecosystems is called the Biosphere and can be termed as the zone where life can be found on earth. It is a closed and self regulating system, apart from extra biospheric radiation and matter colliding with Earth. Organisms have been observed to thrive in space, at least 64km under water, 19 km in the ground and in extreme temperature differences²


1: The Science Asylum (2020), Youtube: What Makes You ALIVE? Is Life Even REAL?!

2: Ball, P. (2004). The Elements: A Very Short Introduction. Very Short Introductions. OUP Oxford

3: Parry, Richard (25 August 2010). "Democritus". Stanford Encyclopedia of Philosophy

4: André Brack (1998). "Introduction" (PDF). In André Brack (ed.). The Molecular Origins of Life

5: Wilkinson, Ian (1998). "History of Clinical Chemistry – Wöhler & the Birth of Clinical Chemistry" (PDF). The Journal of the International Federation of Clinical Chemistry and Laboratory Medicine.

6: Koshland, Jr., Daniel E. (22 March 2002). "The Seven Pillars of Life". Science. 295 (5563): 2215–16. doi:10.1126/science.1068489PMID 11910092.

8: Erwin Schrödinger (1944) in "Whats is Life?"

9: Lehninger, Albert (1993). Principles of Biochemistry, 2nd Ed

10: Luttermoser, 2008 in "Physics 2028: Great Ideas in Science: The Exobiology Module":

11: Boltzmann Ludwig, (1974) "The second law of thermodynamics (Theoretical physics and philosophical problems)"

12: Lovelock, James (1979). GAIA – A New Look at Life on Earth. Oxford University Press 

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15: Pinti, Daniele L.; Arndt, Nicholas (2014), "Oceans, Origin of", Encyclopedia of Astrobiology, Springer Berlin Heidelberg, pp. 1–5, doi:10.1007/978-3-642-27833-4_1098-4, ISBN 9783642278334

O'Neil, Jonathan; Carlson, Richard W.; Paquette, Jean-Louis; Francis, Don (November 2012). "Formation age and metamorphic history of the Nuvvuagittuq Greenstone Belt". Precambrian Research. 220–221: 23–44. Bibcode:2012PreR..220...23O. doi:10.1016/j.precamres.2012.07.009. ISSN 0301-9268.

16: G. Miller; Scott Spoolman (2012). Environmental Science – Biodiversity Is a Crucial Part of the Earth's Natural Capital. Cengage Learning. p. 62. ISBN 978-1-133-70787-5. Archived from the original on 18 March 2015. Retrieved 27 December 2014.
Staff (2 May 2016). "Researchers find that Earth may be home to 1 trillion species". National Science Foundation. Archived from the original on 4 May 2016. Retrieved 6 May 2016.

17: Powner, Matthew W.; Gerland, Béatrice; Sutherland, John D. (14 May 2009). "Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions". Nature. 459 (7244): 239–42. Bibcode:2009Natur.459..239P. doi:10.1038/nature08013. PMID 19444213. S2CID 4412117.

18: King, G.A.M. (April 1977). "Symbiosis and the origin of life". Origins of Life and Evolution of Biospheres. 8 (1): 39–53. Bibcode:1977OrLi....8...39K. doi:10.1007/BF00930938. PMID 896191. S2CID 23615028.
Rothschild, Lynn (September 2003). "Understand the evolutionary mechanisms and environmental limits of life". NASA. Archived from the original on 29 March 2012. Retrieved 13 July 2009.

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20: Hadhazy, Adam (12 January 2015). "Life Might Thrive a Dozen Miles Beneath Earth's Surface". Astrobiology Magazine. Archived from the original on 12 March 2017. Retrieved 11 March 2017.
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See also:


Entropy and Life

Ludwig Boltzmann

Erwin Schrödinger

Albert Lester Lehninger

James Lovelock



Spontaneous generation