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According to Biology Online, life can be defined as "A distinctive characteristic of a living organism from dead organism or non-living thing, as specifically distinguished by the capacity to grow, metabolize, respond (to stimuli), adapt, and reproduce"

If this is the case, would not fire be a living organism? It grows via oxidizing things around it, it grows, and it can adapt, (changing chemical composition based on what it's burning).

Am I missing something? Because it certainly seems that fire is a basic form of life.

  • This is not a definition of life but a heuristic test for distinguishing living from non-living (and some forms of anabiosis would land living organisms on the wrong side of it). Scientific definition of life requires a comprehensive scientific theory of it which we do not currently have, so we can not precisely define what life is any more than what dark matter is. Biologists typically include some sort of capacity for evolution as a likely requirement, see NCBI, but whatever it ends up being fires, stars, and growing crystals will be excluded. – Conifold Nov 26 '17 at 21:24
  • "It is said of the Xanthic sect that they believed fire to be a form of life, since it has the ability to reproduce itself" - a memorable line from a fictional work (Mr Mee, by A. Crumey, 2000). – sand1 Nov 26 '17 at 21:42
  • Ask the Zoroastrians. I think they keep a fire burning in their temples at all times (I think "temple" is the right word). – Gordon Nov 27 '17 at 0:31
  • The spiritual fire that burns the wicked but never the good, is very much alive and so very wise. – Bread Mar 26 at 4:25
  • Fire has pretty much all the characteristics of life. Living things seek to spread, and consume, by converting one thing into another. Both will spread unabated until starved of resources. And if we imagined a nano-machine intelligence which replicated itself by consuming it's surroundings it may appear little different from fire. But biological life requires a specific form of replication which fire does not share. – Richard Mar 26 at 13:09
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Interesting idea, possibly the wrong place to ask that question, but still something I will do my best to answer. Fire is not typically considered a form of life because it is a property of both the thing being burned and the oxygen being "metabolized" to do so. As a result, fire does not technically "adapt", though that could be another definitional dispute. Fire can, and does, change based on its fuel source slightly, but that is not an adaptation to an environment so much as it is a different kind of fire. Typically, one also would require that something be made up (at least in part) of organic matter or a sustainable and tangible form of matter, which fire is not.

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    I initially asked on biology.stackexchange.com but they asked me to move it to this site. – mcchucklezz Nov 26 '17 at 20:29
  • @script8man Hm. Curious, but I can see the thought behind it. That definitely seems like it would be the right place to ask this sort of thing, but take an answer where you can get one I suppose. – Dallas Crenshaw Nov 26 '17 at 20:30
  • Life must not be organic, there is dispute whether there can be silicic forms of life. But then smart enough computers also could be called life. – rus9384 Jun 18 '18 at 13:00
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Probably this is the right place to ask.

Following such definition, some rocks would be alive (they can grow slowly by incorporating minerals on its structure, then when they break, the process can be described as reproduction, since the children still follow the same mechanisms, growing and reproduction, along thousands of years), and can metabolize certain minerals (metabolism: chemical processing oriented to keep existence, then, if some chemical process contribute to enforce the structure of the rock, and therefore its existence and persistence in time, it can be considered metabolism), respond to several types of stimuli, including electric, magnetic, pressure, etc. Fire or bacteria are just faster. Usually, the problem is just scaling: if we would be able to see rocks grow, we would easily exclude the terms grow, metabolism, reproduction from the definition.

The term "life" has been always very complex to precise. We have an intuition of what life should be, characterized by certain types of reproduction, not all, certain speeds of growth, not all, certain types of actions and reactions, not all, certain types of movement, not all, etc. But we have NOT a formal definition of life that can be applied to define if an entity is alive or not. Proof of this is the fact that we still don't know how to classify viruses: they have more similarities to rocks than to living beings, nevertheless they behave exactly like living entities, within scales and levels that match perfectly our intuition of a living entity.

So, life is still a subjective definition, discriminating entities moreover based on their dimensional scales (times, sizes, reactions, mutations), and certain subjective factors like self-generated movement (which rocks or robots can also perform due to environmental changes) or reproduction (which software programs can do perfectly). There is no unique definition of life. Every discipline related to biology could have a different one.

Finally, it's a nice exercise to find definitions of life and submit rocks, fire water drops or clouds to a strict formal test against them. It's easy to find that there are no concrete, formal, strict and objective differences between biologically living and non-living entities.

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Even if you solidify the checklist into a definition, which may not be the intention, you didn't go down it properly.

You skipped 'react to stimuli', and I think it would be hard to claim fire does such. It is a process that spreads by direct contagion. It 'reacts' only to conditions within itself. It does not take input from the environment outside and change according to that data.

Even bacteria and sponges encounter chemicals around them and choose not to take them in, but fire does not. It may fail to spread somewhere, but not in response to anything not already contained within itself.

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    Fire is a system, systems have inputs and outputs, and they change itself, and change the inputs into outputs. Fire will accept a higher level of CO2 or less O2 and get extinguished. Isn't that a reaction to an action? As any thing on the universe, all things follow all causality rules. Perhaps it's difficult to recognize them, but I/O channels always exist (an input channel does not necessarily mean a pipe or a button: a rock inputs potential energy by other means, like a positional change, not pipes or cables). Closed systems are almost impossible on our universe. – RodolfoAP Nov 30 '17 at 10:10
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    "Internal" or "external" actions and reactions are blurry concepts. All systems output external responses to certain inputs (also called actions or stimuli). Since systems are made of subsystems that have similar properties (fire has molecules inside, which also accept inputs and generate causal outputs in response), then "internal" behaviors also exist, but such is a "humanized", biased point of view of systems, kinda granting will to systems. Systems have channels and reactions raise as an answer to actions. That's the definition of stimulus/answer. Should read: causality. – RodolfoAP Nov 30 '17 at 10:22
  • Thanks for the utterly pointless lecture. I am sorry I answered your non-question. – jobermark Dec 1 '17 at 1:54
  • Sorry for that. I was asking if a change in the input causing a change on the system and the output wasn't a 'reaction to stimuli". – RodolfoAP Dec 1 '17 at 2:30
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    So if something hits me going 50 miles an hour and I fall over, this is a reaction to an action, but it is obviously not a response to a stimulus. Concepts can be blurry at their boundary and still separable. Flowing naturally to a lower energy position, like water going over a cliff or fire going to where there is more oxygen is not a response to a stimulus any more than a person's body complying with gravity and force is. – jobermark Dec 1 '17 at 17:58
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Metabolism isn't simply converting energy but rather using energy for other processes, growing, reproduction, movement etc. Also, if a fire is the flame then fire is an epiphenomenon of the chemical process, that is the "metabolism" is not caused by the fire. But if you say the flame is only an effect of the fire, then fire reduces to simply opportunistic chemical reaction.

Grow/Reproduce: this introduces a bit of a Sorites Paradox. When can we say that one fire has become another fire? If it "moves" from the curtains to the ceiling did it grow, or reproduce? Did an ember of a forest fire start a new forest fire or did the first fire heal and regrow? It seems to my that fire can only be said to either grow, or reproduce.

Organisms are sensitive to their environment and react by moving, i.e. there is observable choice involved. Fire spreads, wherever conditions allow, it does not move and it doesn't have the apparatus to choose.

  • I like how you use "choice" to define life. There is some free will in life in general. By this definition even a virus makes a choice--and has enough free will to do so. It would help your answer to provide references to others who take a similar position. This also would allow an interested reader such as myself a place to go to get more information. – Frank Hubeny Jun 18 '18 at 11:26
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It comes down to how you define "life". There is no universally accepted definition. If you adopt the definition given by the link you gave, then you would indeed classify fire as "living" since it meets all the criteria.

Of course most of us would agree that fire isn't actually alive. This just means that the definition needs to be improved. Exactly how though remains a matter of contention (this is why there is no universally accepted definition of life). Still, it's possible. Wikipedia's article on life gives these seven characteristics:

  1. Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
  2. Organization: being structurally composed of one or more cells – the basic units of life
  3. Metabolism: transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
  4. Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
  5. Adaptation: the ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
  6. Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
  7. Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.

Fire fails characteristic #2: it is not composed of cells.

Of course this just leads to, how do we know life is composed of cells? Can we guarantee that if we find aliens elsewhere in the universe that they would also be composed of cells? I think the logical answer here is "no" (we can't even guarantee they will be carbon-based, for example). But does that mean those aliens aren't alive? Again, this is why there is no universally accepted definition of life.

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