Here is something i wrote sometime ago-
Virtually all authors who have considered life from the point of view of molecular biology have regarded the property of self-reproduction as the most fundamental aspect of a living organism. -John D. Barrow and Frank J. Tipler, The Anthropic Cosmological Principle
Well there are problems here since there are easy examples of living things that do not or cannot reproduce, for example childless people and mules; and, there are easy examples of nonliving things that do reproduce, for example crystals and mesons.
They go on with this amendment :
"Since all living things are largely composed of cells which can self-reproduce, or are autonomous single cells with self-reproductive capacity, we will say that self-reproduction is a necessary property which all living things must have at least in some of their substructure"
Hmm..consider this :
"He La" cells from the cervix of Henrietta Lane, a woman who lived in Washington, D.C.-continue to be grown in laboratories around the world, despite Lane's death from cancer of that same cervix in the 1950's. -Lynn Margulis and Dorion Sagan, What is Life?
OK so this lady is definitely not living, but her constituent parts are continuing to reproduce. So HeLa cells are still alive, but Henrietta Lane is not. Thus we must make a distinction between an organism and it's constituent parts
But what about crystals and mesons ?
"Yet we would be unwilling to regard either salt crystals or mesons as living creatures. The key distinction between self-reproducing living cells and self-reproducing crystals and mesons is the fact that the reproductive apparatus of the cell stores information, and the specific information stored is preserved by natural selection...The reproductive "apparatus" of crystals and mesons can in some cases store information, but this information is not preserved by natural selection . . . " Barrow and Tipler
The writer William Poundstone, in The Recursive Universe, derives the following criteria for living things from Von Neumann's work:
(1) A living system encapsulates a complete description of itself.
(2) It avoids the paradox seemingly inherent in (1) by not trying to include a description of the description in the description.
(3) Instead, the description serves a dual role. It is a coded description of the rest of the system. At the same time, it is a sort of working model (which need not be decoded) of itself.
(4) Part of the system, a supervisory unit, "knows" about the dual role of the description and makes sure that the description is interpreted both ways during reproduction.
(5) Another part of the system, a universal constructor, can build any of a large class of objects-including the living system itself-provided that it is given the proper directions.
(6) Reproduction occurs when the supervisory system instructs the universal constructor to build a new copy of the system, including a description.
Now using the above we can eliminate some examples of non-life, including salt crystals, mesons and viruses are considered nonliving because they also do not include universal constructors; instead, they invade cells and commandeer the universal constructors there in order to reproduce
However under this definition, mules would clearly not be living
Poundstone asserts that dead bodies in general should be considered living, at least until decay destroys the internal structures of the cells but see the above point on "He La" cells. For, until that happens, it is conceivable that some bioengineer might clone a new person from a cell of the dead person. Thus, the dead person has reproductive potential, and should be considered living. Reduction ad absurdum ?
Schroedinger :
"What is the characteristic feature of life? When is a piece of matter said to be alive? When it goes on "doing something," moving, exchanging material with its environment, and so forth, and that for a much longer period than we would expect an inanimate piece of matter to "keep going" under similar circumstances." -Erwin Schrodinger, What is Life?
Is flame alive ? Do flames postpone the state of maximum entropy much longer than we would expect a nonliving thing to?
In some way a flame can be considered as self-sustaining, as the fuel burns new fuel is continually being exposed and heated to the point where it can also burn. So by Schrodinger's definition a flame is alive
On the other hand, tardigardes are simple organisms that can be dehydrated into a powder, and which can be stored in this state for years. But if water is added, the tardigardes resume their living functions. When in the anhydrous state the tardigardes do not metabolize. Are they "dead" material during this period?
So Schrodinger says :
Living things are systems with a characteristic order that persists over time.
Living things are active. (Even if an organism appears to be sitting still, processes are going on inside it.)
Living things are open systems that exchange material and energy with their environment.
Living things increase the entropy in the environment around them.
"Every five days you get a new stomach lining. You get a new liver every two months. Your skin replaces itself every six weeks. Every year, ninety-eight percent of the atoms of your body are replaced. This nonstop chemical replacement, metabolism, is a sure sign of life." Chilean biologists Humberto Maturana and Francisco Varela
"An autopoietic entity metabolizes continuously; it perpetuates itself through chemical activity, the movement of molecules. Autopoiesis entails energy expenditure and the making of messes. Autopoiesis, indeed, is detectable by that incessant life chemistry and energy flow which is metabolism. Only cells, organisms made of cells, and biospheres made of organisms are autopoietic and can metabolize." -Lynn Margulis and Dorion Sagan, What is Life?
Margulis and Sagan consider non-replicating examples such as mules:
"Replication is not nearly as fundamental a characteristic of life as autopoiesis. Consider: the mule, offspring of a donkey and a horse, cannot "replicate." It is sterile, but it metabolizes with as much vigor as either of its parents: autopoietic, it is alive. Closer to home, humans who no longer, never could, or simply choose not to reproduce can not be relegated, by the strained tidiness of biological definition, to the realm of the nonliving. They too are alive."
Margulis and Sagan go on to consider viruses:
"In our view, viruses are not [alive]. They are not autopoietic. Too small to self-maintain, they do not metabolize. Viruses do nothing until they enter an autopoietic entity: a bacterial cell, the cell of an animal, or of another life organism. Biological viruses reproduce within their hosts in the same way that digital viruses reproduce within computers. Without an autopoietic organic being, a biological virus is a mere mixture of chemicals; without a computer, the digital virus is a mere program.
Smaller than cells, viruses lack sufficient genes and proteins to maintain themselves. The smallest cells, those of the tiniest bacteria (about one ten-millionth of a meter in diameter) are the minimal autopoietic units known today. Like language, naked DNA molecules, or computer programs, viruses mutate and evolve; but, by themselves, they are at best chemical zombies. The cell is the smallest unit of life."
Now going back to the candle flame. A flame's gasses continuously in motion. New material enters through the bottom of the flame, and waste products such as smoke and carbon dioxide exit through the top. All the molecules in the flame are regularly replaced, yet the flame itself persists. Is the candle flame an autopoietic system?
Let us compare the candle flame with, say, a mouse.
Both the candle flame and the mouse need fuel. the candle uses wax and when this is gone so is the flame. When the mouse runs out of food, it just starts exploring until it finds some.
But the flame can spread to and engulf new fuel, such as when a spark starts a forest fire. But there are limitations to this spreading, such as that fire outdoors will tend to spread downwind. It may be that better sources of fuel are lying upwind, and so never get used. A mouse, by contrast, can move upwind, or uphill. It can see or smell food at a distance, or it can just keep searching around until it finds something.
The mouse doesn't have to feed at every moment, as the flame does. It can store up energy inside and use it to survive until it finds more food.
There are other hazards that can affect the candle flame and the mouse. A candle can not knowingly defend itself, a mouse can run away from danger
The mouse can find new fuel and evade danger. It has a relationship with it's environment. Herbert Spencer's definition of life in Principles of Biology is "the continuous adjustment of internal relations to external relations."
Well I am still no closer to defining life but I will keep searching till..well as John Lennon said " You don't know what you've got until you lose it"
