Life: a complex definition

The concept of life has always been one of the most complexes to enunciate; to give an accurate definition is how to give the exact definition of energy, something almost impossible.


We can say that energy is a physical magnitude that measures the ability of a body or a physical system to do work, regardless of whether such work is possible. We can also define it as the force for displacement or depending on the type of energy analyzed in other numerous ways, chemical energy, kinetic energy, potential, electric, nuclear, etc. So, what is energy? It’s just a mathematical equation? A brick when it falls on your foot does a job, and it hurts. Equations being abstractions shouldn’t hurt. Similarly, to what we can say about the concept of energy, we can conclude that some definitions are particularly complex. In our case, life is perhaps the most complex of all.

The paradox in the study of life

Since ancient times many thinkers have debated what was the true definition of this word. Not only biologists have tried to give a clear and exhaustive vision of it, but also philosophers, mathematicians, physicists, and so on. Paradoxically, biologists often wonder less about the meaning and what life is than other scientists.

They just study it in its most hidden aspects without wondering what it is. I as a biologist often fall into this category, but as soon as the thought falls on its meaning the mind begins to wander among the cosmic aeons in search of a solution. To define the term “life” during the centuries, numerous models have been proposed to define the philosophical and scientific concept of the living being.

The hardest question

According to Aristotle in 350 B.C. life was defined as “a body that feeds and then declines in a defined time”. A very macroscopic definition, valid, but generic. More than two millennia pass in which great thinkers confronted each other and in 1894 Engels (German philosopher and sociologist) arrives who defines it as “the existence of protein structural forms with renewable chemical components”. The scientific advances of the time do not yet include the centrality of nucleic acids and DNA in cellular physiology.

In 1944 Schrodinger (the great Austrian physicist father of quantum mechanics), adds the concept of “laws that order matter” and defines life as “ordered entities and matter subject to an existing order that keeps them in a stationary state”.

A few years later, in 1949, Von Neumann (Hungarian mathematician, physicist and computer scientist), with his fine calculating mind, and even a little visionary, defines living organisms as “simple automata able to replicate”. Almost forty years go by and in 1986, Maynard-Smith (English biologist and geneticist) declares that thinking organisms are “entities with the property of multiplication, variation and inheritance”.

In more recent times

In 1994, Gerald Joyce (a researcher at NASA) defined life as a “self-sustaining system capable of undergoing Darwinian evolution”. This seems to be a satisfactory definition, but a critique can be made of the term “self-sustainable“. Is an organism capable of supporting an autonomous process of energy flow?

Animals as we know them feed on the outside feeding on plants or other animals, plants, seen in a broad sense, feed mainly with the Sun, some parasites, depend strongly on the organism in which they are and so on. By strictly using this definition, many organisms that are also non-living are likely to be considered non-living.

In 2004, Ruiz Mirazo proposed a new definition “life is an autonomous system with open-ended evolutionary capacity”, similar to the previous one, he could be criticized more or less the same. From 2009 onwards, the scientific community chooses a fairly unanimous definition adopted even today by NASA in its space exploration projects; “life is characterized by organized matter that is capable of being subjected to reproduction and natural selection”.

Raffaello Sanzio, School of Athens, debate on life definition
Figure 1 – The School of Athens, Raffaello Sanzio 1509. [Source:]

A possible solution

In this debate that has lasted for millennia, in 2002 Daniel E. Koshland Jr. attempts to close the question about the possible characteristics that an organism must possess to be considered alive by publishing in the well-known journal “Science” an article in which it lists seven possible prerequisites that a physical entity needs to enter the circle of life. To make everything clearer, Mr Koshland imagines that seven fundamental pillars support the vault of a Greek temple to which he gives the name of “Temple of PICERAS“.

The pillars of life

The first pillar (P) corresponds to the term “program“, an entity defined alive must be characterized by something that makes its morphology, behaviour and physiology stable. A program is needed that can transmit genetic information (nucleic acids) for the proper functioning of the whole organism.

The second pillar (I) corresponds to the term “improvisation“, a form of life that must possess a program capable of improvising according to the environment and external conditions without first adapting, the concept of adaptation comes after improvisation.

The third pillar (C) corresponds to the term “compartmentalization“. Life, to originate, must develop in confined spaces. Today, we can observe the cell, which is a compartment bounded by the plasma membrane. However, this seems to be only a finishing point, the starting point was probably in the porous rocks of the Earth’s juvenile stages.

The fourth pillar (E) is energy. Every living organism needs to maintain a flow of energy that corresponds to its metabolism.

The fifth pillar (R) corresponds to the term “regeneration“. The rogram of a living being must be able to repair itself because it is subjected to wear and tear. Concepts of reproduction and duplication derives from this.

The sixth pillar (A) corresponds to the term “adaptation“. Here, the evolutionary process comes into play, the program becomes variable thanks to improvisation, which gives rise to adaptation. An organism adapts to its environment in the best possible way.

The seventh and last pillar (S) can be described as “seclusion“. It is a complex concept that can be summarized in the fact that in living systems there are networks of processes that are always respected in their directionality. From A you get to C through B, with a certain and fair dose of elasticity. Never, if not in times appropriate to evolution, we pass from A to C starting from B.

Conclusions and reflections

If a body has all these characteristics, with a good approximation it can be considered alive for most of the scientific community. These listed qualities do not define the concept of life, but only the characteristics necessary to be part of it.

To date, NASA’s definition is used by the scientific world both to define life on Earth and to define it elsewhere. In astrobiology, this definition is slightly expanded to include all those possible entities based on characteristics completely different from those we are used to.

Original article by Luca Tonietti – Translated by Luigi Gallucci


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  • McKay, Chris (18 September 2014). “What is life? It’s a Tricky, Often Confusing Question”. Astrobiology Magazine;
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  • Jeuken M (1975). “The biological and philosophical defitions of life”. Acta Biotheoretica. 24 (1–2): 14–21.
  • Featured image: – The Seven Pillars of Life. Daniel E. Koshland Jr. Science 22 Mar 2002 : 2215-2216;
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