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José María Díaz Nafría
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La vie
 German Leben
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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.

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” in 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. In a biological sense, only Bacteria and Humans are alive (from this list).

2.     Proposed definitions which are nowadays not considered anymore

One of the first proposed ones is by Greek philosopher Empedocles (495 B.C.)  that everything is made from the four elements: air, water, fire and earth. Life therefore is made from a combination of those. Another famous one is Democritus` (460 B.C.): The essential characteristic of life is having a soul. Like everything else it is made of atoms. But he considered them to be fiery because he saw a connection between heat and life. Aristoteles (384 B.C.) introduced the idea of “Spontaneous generation” which states that life can emerge from non-living things like dust, garbage or even mud. In contrast to this stands Vitalism. Proposed by Georg Ernst Stahl (1659) it states that organic material can only emerge from living things. Both theories were disproven and are nowadays not considered anymore.

3.     Comparing more modern definitions with some examples

Human cells and erythrocytes are considered to be alive, viruses and fire are considered to be not alive. 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, grow, some type of homeostasis, adapt to different diets and reproduction, but is not able to maintain an organized form and response to stimuli. 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.

4.     The Aspect of maintaining order

Schrödinger (1887): “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”. Lehninger (1917): “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 is also stated in the classical physical definition of life: “Living beings are thermodynamic systems with an organized (order) molecular structure that can reproduce itself”. These definitions imply a very high contribution of entropy to be considered.

5.     Entropy as the key

The definition of thermodynamics: “Life is an open system which makes use of gradients in its surroundings to create imperfect copies of itself” keeps its demands relatively low. Boltzmann (1844) made a more charging statement: “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”. Further implying that entropy is the key for dividing the not living from the living. Loveclock (1919), when asked for how he would search for life in the universe, even went as far as to say: “I`d look for an entropy reduction, since this must be a general characteristic of life”.

6.     Combining the learnings

The most obvious trait life must have, is being able to convert energy, 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.


What is life? (1944) by Erwin Schrödinger