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The prevailing biology of the modern era describes life as a system. A system is defined a set of things working together as parts of a mechanism or an interconnecting network. The NASA definition of life is this: “Life is a self-sustaining chemical system capable of Darwinian evolution”

However, I think there is a problem with this.

A living thing is understood as a being whose parts work together for one goal, which is the sustainment of the whole organism. In this sense, the parts comprise truly one being, as this principle that unites the parts is intrinsic to the organism.

However, a machine is not a one being as much as a heap of sand is not a one being, as its goal, function is imparted from the outside. Its principle of unity is extrinsic, its unity is in the perceiver's mind, not in-itself.

Therefore, we can say that a machine is only a metaphor, something that resembles life but not quite. Machine or a system is built to mimic life.

If this is the case, isn't defining life by something that mimics life problematic?

Chanhyu Lee
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    Parts of an organism do not work together for one goal, they just work. The sustainment of the whole organism is a side effect of selection, those organisms that did not manifest it did not last long enough to be around either. Making it into a "goal" is just an anthropomorphic shorthand for ease of presentation. The only difference between organisms and self-sustaining machines in this regard is that what takes millennia to select takes much less to produce artificially. – Conifold Nov 26 '22 at 05:21
  • Then you are simply saying that there is no essential difference to what is living and what is not living. This is because you think system is a higher category that contains both living ones and non living ones. And I am saying that to define life as a 'kind' of system is not thinking about the real distinction between organisms and man-made systems. This may be a case of gerrymandering. I think the concept of system itself is not a natural kind but a human convention, but life is not a human convention. @Conifold – Chanhyu Lee Nov 26 '22 at 23:25
  • Not at all. There may well be a metaphysical difference between life and non-life (for example, immaterial *elan vital* of old or whatnot), but "intrinsic goals" are not metaphysics, they are teleology. Ascribing goals to something does not help defining what it is, even when those goals are declared "intrinsic". Living organisms are almost certainly a natural kind (if those exist at all, which is controversial), but whatever their "real distinction" is it has to be sought elsewhere. Nor does this preclude manufacturing of artificial "systems" that are man made exemplars of this natural kind. – Conifold Nov 27 '22 at 08:16
  • But teleology *IS* an important concept in metaphysics. Aristotle counts it as one of the category of causality. https://www.researchgate.net/publication/304875436_Synthetic_Life_and_the_Bruteness_of_Immanent_Causation Modern literature also deals with it. @Conifold – Chanhyu Lee Nov 28 '22 at 04:31
  • Aristotle's "causes" do not mean what "cause" means today, they are [reasons/explanations](https://plato.stanford.edu/entries/aristotle-causality/#FouCau). And that is the place of teleology. Metaphysics comes first, then suitable parts of it, if any, can be rephrased as teleology. But anthropomorphic explanations (which final causes are in Aristotle) are not a good way to phrase definitions of natural kinds. – Conifold Nov 28 '22 at 12:40
  • a) Causality is the habit of a consequence following a cause. Teleology can be interpreted as the opposite: the quest of a cause that will trigger a consequence (the goal). Ergo, both are the same, just describing causality from both ends. b) The definition of system depends on causality: relations, as in "a system is a set of _interrelated_ parts", are persisting interactions, that is, repeated actions followed by causal reactions (is there any other kind?), that is, causes and consequences, that is, causality. – RodolfoAP Dec 30 '22 at 09:25
  • Relevant: https://philosophy.stackexchange.com/a/37398/17209 – Philip Klöcking Dec 30 '22 at 10:17

5 Answers5

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The definition of life

Something is alive when it can reverse entropy locally. Things that are alive must eat to sustain themselves; this process reverses entropy, as a disorganized world becomes re-organized as this living thing.

However, the entropic books have to balance; the whole process cannot go backwards. So the living thing reverses entropy only locally; that is, within the living thing itself. As for the rest of the universe, entropy ran a little faster.

Mark Andrews
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    This is a naive definition. Cosmic systems (e.g. a star) form due to random events that lead to a "reduced local entropy" system compared to the previous state. Entropy then increases until eventual dissipation (back to max entropy). And a star is not considered a living system. There are plenty of equivalent examples where natural systems raise "reducing local entropy", and then increasing it until dissipation without being alive. If entropy would never "reduce locally", nature would not exist. And nature is not a set of "living" systems (in a biological sense, as the question requires). – RodolfoAP Nov 26 '22 at 09:09
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    Notice that entropy decreasing internally to open systems is a common fact (e.g. a rock that enforces its internal structure, by incorporating minerals from the water, which is exactly like eating, like plants or dogs get minerals, a condition you require), and it is allowed by the 2nd law, because such law applies only to closed systems. And rocks are not considered to be alive. Please don't say that living systems must chew what they eat to be considered alive. – RodolfoAP Nov 26 '22 at 09:25
  • @RodolfoAP. Thank you. This comment is food for thought. – Mark Andrews Nov 26 '22 at 21:12
  • Absolutely not. Locally decreasing entropy is routine for many things we don't consider alive. Locally decreasing entropy is routinely not part of things we consider alive. – Boba Fit Nov 29 '22 at 16:34
  • @RodolfoAP a rock would not disintegrate, if it stops absorbing minerals (an simpler analogy that you could have used is crystal growth). Living systems are inherently unstable - they cannot exist, unless they continuously work on reducing entropy. Furthermore, a rock is not doing any work when growing - this is how it is different from, e.g., a car engine. In this sense, yes, one has to "chew" to be considered alive. What is really missing in this definition is replication... but if we define living systems as replicating ones, this means that an animal taken alone is not alive ;) – Roger Vadim Nov 30 '22 at 08:25
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The definition of life is based on two features, each of which requires some technical description. These are: reproduction and irritability.

Reproduction refers to the creation of not-necessarily perfect copies. There are, of course, many patterns of reproduction. It can be through dividing into copies, or through sexual reproduction. Other means are imaginable. Prions give an example.

Irritability refers to responses to outside stimuli. A non-living thing will respond "mechanically." If you tap a rock you get the same response every time. If you tap a living thing it may adjust its response after it adjusts to the taps.

There is a thing they show you in high school biology where some single-cell critters are exposed to a mild vibration. The first time they all contract to try to protect themselves. After several repeats they get used to it and stop contracting.

A chemical process will respond the same way to the same conditions. A living thing will respond differently depending on the organism's internal state.

Deciding if a thing is alive or not is not trivial, particularly at the boundary. Things such as a virus, whether biological or computer, are right at the boundary. There are additional examples that are slightly on either side of this towards or away from things we consider living. For example, prions are misfolded proteins that can reproduce and produce prion-based disease, and some of those diseases can be spread even between species. There is disagreement over whether these examples are alive.

Boba Fit
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From the physical point of view, living system is a heat engine: it takes energy from outside world, builds itself and replicates (which means that it locally reduces entropy), and rejects the unusable energy to the outside world.

What it makes different from machines or other natural phenomena that locally reduce entropy is the replication, in which imperfect copies are created (mutations), which means that over many generations the system can adapt to the environment - in being more efficient in extracting energy, building itself faster and ultimately creating more copies of itself (in biology fitness is defined as the number of copies produced per unit time.)

What makes it different from other systems that may produce (imperfect) copies of themselves - like viruses or computer code - is that it possesses everything necessary for extracting energy and building/reproducing, whereas viruses and computer code are dependent respectively on their host and hardware in providing support for their functioning (biologically viruses are classified as obligatory parasites - they cannot function without a host.) It is in this sense that biologists define cell as an elementary unit of life - excluding sub-cellular creatures like viruses, prions, viroids, etc.

Remarks:

Roger Vadim
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  • "Heat engine" is a specific thing, not correctly applied to organisms. An organism, neither it's metabolism nor reproduction, is absolutely *NOT* defined by locally reducing entropy. You should include some discussion of disagreement about whether viruses and prions are living. – Boba Fit Nov 30 '22 at 20:35
  • @BobaFit heat engine is a very general thermodynamic concept, despite suggestive name and centuries old examples given in textbooks. – Roger Vadim Nov 30 '22 at 20:47
  • Quite a biased and subjective definition: "may produce (imperfect) copies of themselves": strictly, all copies are either identical or not. A child is not an "imperfect copy" of its mother; in any case, self-replicating robots fulfill this definition (they are copies, they generate heat), and are not considered biologically alive. Even some types of rocks fulfill this definition (yes, they convert energy, yes, they grow in rivers accumulating minerals, they break and their "child" grow and follow the same dynamics, even keeping similar oval forms). – RodolfoAP Dec 30 '22 at 09:31
  • @RodolfoAP *quite a biased and subjective definition* is unfriendly/unkind/condescending attitide - hence the flag. Otherwise, the examples in your are rather disingenuous and are easily refuted by carefully (re)reading my answer. – Roger Vadim Dec 30 '22 at 09:43
  • @RogerVadim please don't take it personal. One more thing: a _heat engine_ is a thermodynamic system, a concept depending on the laws of thermodynamics; BUT thermodynamic laws (e.g. the 2nd law) do not apply to open systems. So, it is logically inconsistent to describe open systems as closed systems / thermodynamic systems / heat engines. – RodolfoAP Dec 30 '22 at 12:15
  • @RodolfoAP you are mistaken, thermodynamic laws do apply to open systems - one routinely distinguishes *system* and its surroundings (*thermostat/reservoir*), with which the system may exchange energy and particles. These distinctions explicitly appear in the second law. – Roger Vadim Dec 30 '22 at 13:43
  • @RogerVadim "thermodynamic laws do apply to open systems": common elementary misconception. Entropy is the energy dispersal between the subsystems of a thermodynamic system that obeys the 1st law. The "surroundings" are those of the subsystems, not of the system. When law 1 breaks, 2 and 3 break as well, which makes the container not anymore a thermodynamic system. You can calculate the entropy of a container with a grenade inside, before and after the explosion. What you say implies that if the explosion splits the container in two halves, internal entropy can still be calculated. – RodolfoAP Dec 30 '22 at 16:11
  • @RodolfoAP I think you are a bit confused about the basic concepts of thermodynamics and statistical physics. But I prefer not to discuss it here - it is better to do this in the physics SE. – Roger Vadim Dec 30 '22 at 16:54
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I think you have imagined your objection to the NASA definition. That definition is that life is not merely a system, but a 'chemical system capable of Darwinian evolution'. Therefore the definition is not relying on, and should not be mistaken for, an analogy with a machine.

Marco Ocram
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Perhaps a list will aid us in defining life

100% certain Non-life: The Rock of Gibraltar, water, oxygen

Not a 100% certain whether life/non-life: Rabies virus

100% Life: Scottish terriers, caymen, ostriches, poison dart frogs, tuna, oaks, toadstool, Douglas firs, lotuses, E. coli.

Where exactly is the glitch?

Agent Smith
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