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My attempt at shortest definition of the term 'life'

  1. Apr 11, 2009 #1
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    Life is an ability of matter to manipulate itself.

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    So what do you think?

    PS: originally I thought it up and wrote it here: https://www.physicsforums.com/showthread.php?t=302477&page=2
     
  2. jcsd
  3. Apr 11, 2009 #2

    Evo

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    That wouldn't work.

    Dictionary definition of manipulate.

    Merriam Webster

    1: to treat or operate with or as if with the hands or by mechanical means especially in a skillful manner
    2 a: to manage or utilize skillfully b: to control or play upon by artful, unfair, or insidious means especially to one's own advantage

    3: to change by artful or unfair means so as to serve one's purpose
     
  4. Apr 12, 2009 #3
    Well, here it would mean 'to manipulate' in some broader meaning. ...I mean - what other word... ...'to reorganize' ...'to structure'

    'To manipulate' is closest to defining the concept - property of mater to have mechanisms to change properties of itself (and through it even affect its environment, but - in order to do so it first must 'manipulate' itself for such task)...

    I used term matter not substance, because - maybe there is life which is made of physical fields...
     
  5. Apr 13, 2009 #4

    Moonbear

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    Is there some reason to prefer short over accurate? If you need to redefine the words within your definition in order for it to make sense, perhaps it's not a very good definition.
     
  6. Apr 14, 2009 #5
    That's not always the case. Mathematicians often change their definitions around in order for some desirable property to be true.

    Actually, this happens in all fields. For example, in Biology it is common to use the word "gene" to refer to a protein-coding stretch of DNA. This definition of gene as something that applies to DNA was certainly not how Mendel would have defined it.

    You might say that "a piece of DNA with certain properties" is not the definition of the word gene, rather a fact that we learned about those things that satisfy Mendel's definition is that they are also pieces of DNA with certain properties. My point is just that now that we know this fact we can just as easily reverse which one is the definition and which one is the fact.

    Mathematicians play these sorts of games all the time. There often turns out to be a particular way of defining your terms that reflects a useful way of conceptualizing what is known about an object. This might not be the same for all problems. So you might change your definitions from day to day.

    This is just what Biologists are doing when they drop the Mendelian inheritance from their definition of gene and define it to be a piece of DNA. Without that semantic shift if would have been much harder to hypothesize about things like epigenetics (and thus much harder to discover them).

    Of course, with regards to the original poster in this thread... he or she has yet to explain to us why these new definitions are useful...
     
  7. Apr 14, 2009 #6
    It's tempting to devise a shortest possible definition of a concept so common to us as 'life'.
    It's more 'ingenious' to have one sentence as definition than a list.

    What is the list of things wrong with it?
     
  8. Apr 15, 2009 #7
    A self-contained, metabolizing, reproducive system capable of evolving.
     
  9. Apr 15, 2009 #8
    Some hypothetical live being (I mean Universe is surely very imaginative, maybe we can't even fantasize about what nature can come-up with) - it would be live being even if it doesn't reproduce or evolve; just simply: is.
    I tried to stick to that MOMENT when you see it for the first time and say: "Wow, is it alive?" - so, for that moment: what one defines as 'life'.

    We mainly think of life in our way (the way Earth biology functions), but maybe there are, for example, beings similar to atom - their biology would be subatomic, maybe they would even be intelligent (or maybe just like animals), they could feed by decomposing regular atoms and incorporating their subatomic particles into themselves, having nuclear proceses which give them energy, move by manipulating electromagnetic fields etc.
     
  10. Apr 17, 2009 #9
    We can easily build a machine that manipulates itself and there are countless examples of this already. In fact, based on my vague recollection of the definition of life used in biology, I'd reckon we could construct a machine that satisfies all those criteria as well and it wouldn't even be that difficult of a task. I also remember hearing that being primarily composed of only a few specific elements like carbon, hydrogen, oxygen and a few other less popular ones could be a potential criterion, which is again unsatisfying because, as an enlightened previous poster mentioned, there may very well be things in the universe we would consider as alive but not be composed of these elements. Moreover, I think we are at the point, or at least close to it, where we could make machines composed of these elements that also satisfy all of the other criteria.

    The only definition of life that would be satisfying (in some sense) would be a very long one that conforms to all our traditional ideas as to what a living thing should be. That is, our instinctual interpretation of what being alive means is so inelegant that a "satisfactory" definition would be on the verge of being a very long list of things we consider alive and this would include all theoretical living things as well (so this might not even be finite, depending on our imagination).

    The point is we don't have a conclusive definition for life because there isn't one. Life is a theoretical construct that is fallacious by its very nature because the line between living and non-living is arbitrary and subjective, and indeed there isn't really an objective difference between the two (unless you're religious, and then defining what is life is trivial - but it won't be scientific).

    I don't think we should worry about the definition of life - it's far from essential to the field of biology and I'd even dare to suggest its completely worthless (except for when you want tell people that biology is 'the study of life', but let's face it, no academic field is anywhere near well defined as to what area of knowledge it is supposed study, with perhaps the exception of math).


    P.S. I would argue that everything said above also holds if you replace living with conscious/sentient and non-living with not conscious/sentient.
     
  11. Apr 22, 2009 #10
    ...But we are also made of regular chemical elements - the same "dead" ones machine or a self-replicating molecule we make could be made of. If it doesn't truly manipulate (change position or chemical/electric/(etc.) composition) by itself (i.e. instead it manipulates by sequence of commands someone else made, or randomly) it isn't alive - only artificial intelligence or other means of self-manipulation would mean it's alive.
    If we would genetically engineer from scratch some new complex form of life - it would be alive - never mind it's artificial (i.e. we made it). Of course, life from which we came to be became on its own by chance (through infinite-like number of occurrences - most of them without life as result), but nevertheless - if life form from which Earth life came to be was artificially engineered - what - would it mean we are not alive? (my point: same is with machines we make - if their way is self-manipulation - then they are alive (for an artificial intelligence only it's hardware is artificial - a microprocessor)).

    "Life is an ability of matter to manipulate itself."

    Plants are 'alive' - they don't have intelligence, but they are matter which self-manipulates.

    Rocks which move in space and chemicals in chemical reactions (i.e. 'changes') are 'dead' - they don't self-manipulate, they are simply subjected to laws of physics and chemistry - at them there is not that closed loop - 'matter affecting itself' - only that one way deal 'laws of universe affecting matter'. Of course, matter which affects itself does so tanks to laws of the universe, but my point here is: it just becomes a tool here, not the 'perpetrator' itself...


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    PS: imagine life form that was conceived inside a star - some physical fields were tangled in such a way they hold themselves together - substance created it but it is not substance, just some interesting formations of physical fields in space* - random phenomenon (just like molecules which self-replicate (like DNA)), it senses its surroundings and beyond, it grows in knowledge, it moves around, it can manipulate substance etc.
    (* space must have its structure - it's not empty -- natural magnet reorients that structure - we know that (it doesn't make its constituents change position (that would mean some energy), only reorients them))
     
    Last edited: Apr 22, 2009
  12. May 8, 2009 #11
    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"
     
  13. May 8, 2009 #12
    My attempt at shortest definition: "Life isn't."

    Seriously, aren't we enlightened enough to stop and recognise there is no physical justification for the distinction? How many counterexamples does it take? (Shall the next thread be on which tissue houses the soul?)
     
  14. Jul 22, 2009 #13
    I would argue that there is not an objective reason to make any distinctions. The act of separating things into parts that we can reason with is something we do because it is useful to us in some way. Outside of a particular frame of reference that gives meaning to any particular distinction, that distinction is completely arbitrary and subjective.

    This does not mean the distinction made is meaningless. On the contrary, it is completely valid within the frame of reference it is made. As, by definition, there can exist no frame of reference outside of some intelligent agent to create that frame of reference, the only thing here that is truly meaningless is the assertion that there is no objective reason to make distinctions.



    As for a clear and concise definition of what life is? I would say:

    Life is a self-contained, self-organizing, and self-sustaining system that decreases its entropy by increasing the entropy of an external system.


    AFAIK, life is the only system capable of decreasing its entropy that also has the three prior qualities. I see no reason to rule out non-molecular structures or non-reproductive lifeforms. I also see no reason to bar the possibility of a program or AI being considered life.
     
    Last edited: Jul 22, 2009
  15. Jul 26, 2009 #14
    My attempt... "Life is failure of being dead."
     
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