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?

  • 1
    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, 2022 at 5:21
  • 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 Nov 26, 2022 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, 2022 at 8:16
  • 2
    But teleology IS an important concept in metaphysics. Aristotle counts it as one of the category of causality. researchgate.net/publication/… Modern literature also deals with it. @Conifold Nov 28, 2022 at 4:31
  • Aristotle's "causes" do not mean what "cause" means today, they are reasons/explanations. 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, 2022 at 12:40

6 Answers 6


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.

  • 2
    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, 2022 at 9:09
  • 1
    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, 2022 at 9:25
  • @RodolfoAP. Thank you. This comment is food for thought. Nov 26, 2022 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, 2022 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 V.
    Nov 30, 2022 at 8:25

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.


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.


  • "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, 2022 at 20:35
  • @BobaFit heat engine is a very general thermodynamic concept, despite suggestive name and centuries old examples given in textbooks.
    – Roger V.
    Nov 30, 2022 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, 2022 at 9:31
  • @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 V.
    Dec 30, 2022 at 9:43
  • @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, 2022 at 12:15

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.


Is the universe alive?

If your answer is yes, then wouldn't it follow that everything in the universe is alive also?

Some of us living beings can move and change in a time scale we can see ... other things, like the earth, change and reacts to stimuli in a time scale we can only deduce and imagine ...


  • Welcome to SE. This answer is an interesting deduction and observation about how the world works. But I'm afraid it isn't obvious to me how it answers the question. Perhaps you should expand it a bit.
    – Ludwig V
    Sep 17, 2023 at 18:02

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?

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