The definition of Life

In the middle of the 20th century a biologist Tibor Gánti decided to establish theoretical biology following the sample of theoretical physics. He realized that although it seems to be unbelievable, the science investigates the origin of the life though it does not really know, what is it, what is investigated. So, for first he started to work on the definition of the life, he worked on the definition of the living organisms. Then based on it he developed the theory of “chemoton”, a model of the minimal nature of life. He first published it in 1971. On his misfortune it was in Hungarian and his book was published in English only 32 years later:  The Principles of Life, by Tibor Gánti, Oxford University Press, 2003.

According to Gánti the necessary and sufficient conditions to consider something as living are the followings:

Real (absolute) life criteria

  1. Inherent unity
  2. Metabolism
  3. Inherent stability
  4. Information-carrying subsystem
  5. Program control

Potential life criteria

  1. Growth and reproduction
  2. Capability of hereditary change and evolution
  3. Mortality
A bee on a flowering tree. There is every pixel a part of a living organism on the picture.
But what is the definition of life?
Life is everywhere in every pixel of this picture

The absolute life criteria are those, which are when fulfilled by the individual, it is considered as living. In the absence of any of them the system – the individual – is not living but lifeless, or capable of living, or dead. Lifeless means that the system is not organized on that way, which enables the life, e.g. a stone. Capability of living means, that the organization for living is given, but currently the system is not functioning. Like for example a dormant seed. Dead system means that the system was living, but somehow it lost its’ stability forever. Now it is already not able to fulfil the life criteria.

The potential life criteria are not necessary for the life of an individual but needed for the existence of life as a phenomenon. Some individuals at least in certain phase of their life must accomplish these criteria.

We can say that the absolute life criteria represent the definition of the living while the potential life criteria are the definition of the existence of life.

The Cooperation is the essence of life

1. Inherent unity:

“A system is regarded a unit (a ‘whole’) if its properties cannot be additively composed from the properties of its parts, and if this unit cannot be subdivided into parts carrying the properties of the whole system.
A system forming a unit (unit system) is not a simple union of its elements, but a new entity carrying new qualitative properties compared with the properties of its parts. These new properties are determined by interactions occurring according to the organization of the elements of the system. Only the system as a whole displays the totality of these properties.”
Gánti (2003)

Of course, these interactions what Gánti wrote about are not the interactions of “enemies”, not interactions like fight. These interactions do not depress or eliminate the other party. Their purpose is to create new, higher level properties, a new unit by a quality organization of the elements. These set of positive interactions is called by other word cooperation. The definition of the life includes the cooperation of the components of the living organism.

The first life criterium means, that if a set of certain elements cooperate, provide each other with mutual advantages and create a higher level of unit system, then this new unit may be considered as living. (If the other life criteria also fulfilled.)

2. Metabolism – Functioning:

The purpose of metabolism in the living organism is double. On the one hand it produces energy for the system to keep its homeostasis. On the other hand it provides the system with external material for the controlled growth and development of it. Both purposes are listed among the life criteria, so metabolism “only” supports other life criteria.

By the way it is theoretically imaginable that the energy transfer which enables the living system to keep its’ stability comes on other channel than metabolism. Imagine for example the energy transfer through electromagnetic radiation. Theoretically the reproduction of the living system can take place also outside the parent system. Gánti in his definition of the life used the expression “metabolism”. I suggest “functioning” instead of “metabolism”, because it enables more general definition. This term is not directly related to the form of life which present on earth.

The life on earth is based on chemical processes, and Gánti investigated the boundary between living and nonliving, between a biological system and a “simple” chemical system. From the point of the view of a biologist or a chemist – considering the forms of life on earth – metabolism of course is a very important life criterium.

A dried seed is not considered as living rather as viable. It does not perform metabolism although fulfils all the other life criteria. The dormant seed is not living only viable because it does not function.

So more generalized we can say that functioning is this second life criterium. This term also fits for extraterrestrials from another galaxy who “eat” gamma rays and do their kids in workshops based on templates.

Functioning is a supportive life criterium, which distinguishes the living from viable. It means that a living system is not only constructed so that its components are able to cooperate, but the components also do cooperate. 

The living system is a functioning system.

3. Inherent stability:

The living systems have regulated processes and these processes, these regulations are continuously struggle on keeping the internal environment of the system stable. It means that the processes let only small deviations from the optimal internal conditions of the system.

With the words of Gánti: “It is a special organizational state of the system’s internal processes, which makes the continuous functioning of the system possible and remains constant despite changes in the external environment. It means that the system as a whole, although continuously reacting via dynamic changes occurring within the living system, always remains the same.”

The before mentioned property of a living system, the tendency to regulate and maintain its’ internal environment in a stable state is called homeostasis. “Inherent stability is something more than homeostasis, since homeostasis follows from it. Inherent stability is an organizational property…” Gánti, (2003). So Gánti differentiated the two life criteria – metabolism and inherent stability – and made them logically distinct. The composition of the two is the functioning inherent stability: the homeostasis. On this way he made his definition of life more impressive.

The dormant seed has inherent stability because its’ internal structure makes it capable to maintain its stability. But this dry seed is not in homeostatic state because actually it does not maintain its’ stability, actually does not function.

When someone dies, his tissues, his organs can survive. The tissues can perform metabolism in an appropriate nutrient solution. We can transplant these organs into another person and the organs live there for further years or even for decades. But this does not mean that the dead person did not really die. The person’s body as a unit finished functioning. The conditions for the stability of the tissues and organs are provided externally, the body itself is no more inherently stable. The person is not a stable unit anymore only some components of his body can survive as a part of another unit. The unity of the body is disrupted because the inherent stability of the body was disrupted.

From the point of view of the components of the system the inherent stability is the advantage with which the living system provides them. This is the goal of the cooperation: there are individual entities, there are subsystems, which create a unit system by cooperation and the goal is to create a stable, comfortable environment for the participants of this cooperation. If there were no advantage, no goal, then there were no reason for cooperation.

4. Information-carrying subsystem

A system is considered as alive if the inherent unity and the functioning inherent stability of it is not accidental, it is not a consequence of random events. There must be information stored about its’ structure, regulations and processes which makes them repeatable and reproduceable. This information must be a subsystem of the whole, the living system must contain the information about itself. Not like a building’s blueprint on the table of its’ architect whilst refers to the building, created and maintained independently. The information carrying subsystem of a living system is the integral part of it. The reproduction of the system also reproduces this subsystem, the offspring inherits it.

The information carrying subsystem doesn’t totally disclose the randomness from the lifecycle of the living systems. During the copying of the information random errors can occur. These random errors are the basics of evolution. These errors – as they are random – sometimes result an advantageous trait in the offspring, and sometimes result handicap. The natural selection evaluates the new traits, letting survive or even penetrate those traits which are advantageous for their owners in the actual environment.

5. Program control

The program control is something else than the regulation which ensures the internal stability of the system. The system may deviate from its’ balance to both directions and then due to the regulatory processes it will return to its’ balanced state. Although the regulatory processes are generally not bidirectional, the change of the status of the system is always bidirectional unless it is dead. The program control is about the unidirectional, irreversible changes in the state of the system like growth, multiplication, differentiation, development, reproduction. These processes are also encoded directly or indirectly into the information carrying subsystem of the living organism, and the coded program is executed during the life of the organism. This means another type of control than the regulation: it is a guidance, a navigation through the lifecycle.

My opinion is that the program control is rather a basic condition for the potential life criteria. You can consider an individual as living even it doesn’t grow, develop, reproduce at the moment just exists. Therefore this criteria rather belongs to the definition of life and not to the definition of living.

Potential life criteria


Let’s start at the end. Gánti had long fought about the mortality as a life criterium until at last he considered it as the third potential criterium. We would like to further debate with his decision. Lot of unicellular organisms practically do not die. The end of their life cycle is not the death but the division of the cell. Of course, they can die because of an external shock, but this is not deterministic. And by the way it is a very trivial way of finishing the existence to be eliminated by a much bigger force. So, this kind of mortality means nothing, because everything in the universe shows this kind of mortality.

If mortality means the deterministic one, in the sense that a living system is designed for limited lifespan, which is ended by the death, then it is not true for every living system. The lifespan of some of them does not really end: for example ends with a division. Therefore in our definition the (deterministic) mortality is not even a potential life criteria.

Growth and reproduction

The reproduction is mandatory for the existence of life because even if the mortality is not programmed in every living organism the accidental death is unavoidable. In that way – according to the rule of big numbers – sooner or later every organism will die. So, without reproduction the existing life would disappear.

Growth and reproduction are very closely related features of living organisms. Cells for example divide when they overgrow their regular size. Without growth there is no division, there is no reproduction. On the other hand, the reproduction of a multicellular organism generally starts with a fertilized egg. The growth of the organism starts by the division of the egg cell. After a sequence of multiplied divisions, the organism reaches its’ regular size and maturity, which means the end of the reproduction cycle.

Capability of hereditary change and evolution

Theoretically the generations of living organisms can continually reproduce identical off springs, and under constant circumstances they can sustain this status. But practically the circumstances are not constant. There is a range of the conditions (e.g. temperature, radiation, presence of substances) in which the individual organism according to its’ regulation processes can maintain its’ stability. However, when the alteration of the conditions exceeds this range, then the organism itself must change for the sake of survival, the regulation is not enough anymore.

Because of the randomness of inheritance every individual organism tolerates differently the change of circumstances. For example, one can easily tolerate higher temperatures and cannot survive cold, while others prefer cold and get dry in hot. This diversity enables the species to survive even if the circumstances are changing. The original individuals could not survive through generations, but the current generation with modified hereditary traits can adapt to new circumstances.

There is another important result of hereditary change and evolution. It is hard to imagine that more complex forms of life – like mammals – could come into being in short term. But in a long evolutionary process they developed indeed. The complex forms of life are the results of the evolution. The three driving forces of the evolution: the hereditary change, the competition between individuals and species, and the cooperation between individuals and species.

Life criteria from the cooperation’s point of view

Let’s see our interpretation of Gánti’s life criteria:

Real (absolute) life criteria

  1. Organized for internal cooperation: Inherent unity
  2. Functioning cooperation
  3. Provide with the advantages of cooperation: Inherent stability
  4. Information-carrying subsystem about the organized cooperation

Potential life criteria

  1. Program control
  2. Growth and reproduction
  3. Capability of hereditary change and evolution

Why are these life criteria so imposing?

They are distinct, full and minimal. Therefore these logically defined traits make an imposing set of life criteria . At least for theoretical scientists.

Distinct means that there are no dependencies between the criteria from each other. The fulfilment of one says nothing about the fulfilment of the others, and vice versa.

The criteria are also full, which means that if an individual fulfils the criteria then it is surely living. No other criterium have to be fulfilled for life.

Minimal means that it is the minimum set of the criteria to be fulfilled for life. If any of them is missing that means that the examined item is not a living one.

There is one more beautiful treat of these criteria: they are also general. They do not say anything about the life which is existing on Earth. They do not say anything about carbon chains, proteins, DNAs. About water, oxygen or temperature. If anything in the universe start to show these criteria, then it should be considered as living. It can be an organized self-reproducing beam set of radio waves, an organized plasma entity inside a star, or cooperating gravity waves. It can be any materialistic or nonmaterialistic thing. Fulfilling the above mentioned life criteria is the universal definition of Life.