[lawpoop]

There was a definition I encountered in high school, which I never found a source for, but seems to be pretty robust and address many of the non-living things other definitions include.

This definition is a rubric of several qualities:

1. Metabolizes energy

2. Stores Information

3. Self-replicates

#1 rules out crystals; they are formed by outside forces, they don't metabolize themselves. It also rules out viruses and prions; they don't ingest any "food" to perform metabolic activity. #2 rules out fire; it doesn't store information in a DNA-like molecule or anything simliar. It's purely a chemical reaction. #3 is the obvious thing that differentiates non-living things like rocks from plants and animals.

[benlivengood]

There are still some cool edge-cases. Is a sperm cell alive? An X-chromosome sperm (do X-chromosome sperm have a (probabilistic N)-stage lifecycle?) Are red blood cells alive? Is every molecule of an organism that's alive also considered alive, or just the ensemble? If only the ensemble, are mitochondria alive?

[dandanua]

Also, why we consider a single human as living organism, but not a society? Single human without society is a pitiful and helpless chunk of meat, who unable to replicate btw.

[ealloc]

Sperm cells can be seen as a haploid phase of many organisms' lifecycle, so are alive by pretty much any definition.

In humans, the haploid phase of the lifecycle is single-celled, while the diploid phase is multicellular. In contrast, in mosses and fungi the haploid phase is multicellular while the diploid phase (sporophytes/zygote) is single-celled.

Red blood cells are discussed in the article.

[gus_massa]

> Is a sperm cell alive?

The unfertilized eggs of bees become drones, so they must be alive. As a side effect, this indirectly make bee-like insects evolve a lifestyle with a big colony with a queen.

In fungus, most of the life is as haploid (i.e. a single copy of the chromosomes, like sperm and eggs) instead of diploids (i.e. two copies of each chromosomes, like most of our cells.)

In some ¿unicellular organism? [I can't find a good link now] the haploid and diploid versions are almost equal.

[And plants are also weird, some have 4 or 6 copies of the chromosomes instead of 2.]

[SomewhatLikely]

Self-replication doesn't seem necessary. A human who is infertile is clearly still living. Worker ants who can't reproduce would also be well within the living side of the spectrum to me. I give these examples because it seems to me the definition of living should apply at the individual level, not the species. Individuals after all are the ones doing the dying.

[patcon]

replication is still happening in the lower cellular/molecular scales, even when the aggregate organismal level isn't reproducing that pattern. People who take these definitions very seriously aren't implying that elderly people aren't still alive :)

[mycall]

The infertile human still came from some fertile being.

[bordercases]

What about cloneability?

[SigmundA]

Self replication of information with mutation and evolution would be my definition, which would include viruses, but not crystals, AI could be included if it could evolve on its own.

I think in reality like most things its not black and white, there is a continuum of life and some things are more alive than others.

Edit: perhaps adapting to environment through mutation/evolution or learning or both might be better.

[n4r9]

Evolution has led to life but I'm struggling to see why it should be a criterion. What if something happened that stopped humans evolving, and we maintained the same general genetic distribution for the next few millennia? Would we stop living?

[thibauts]

I’d say life is defined by identity (a bunch of information defining a structure), or a self, and the processes that allow this self to stay identical to itself in time. Something like homeostasis at the organism level. #1 and #2 would be utilities to realize this, and not necessarily integrated. Arguably #3 isn’t necessary as others have said and could be seen as only one manifestation among others of the self-preservation process. A trivial illustration of this definition on which we can all agree is death, where self-preservation processes break and identity disintegrates.

Interestingly this definition would encompass countries and probably any social group as long as they have a name, an identity and processes to maintain it. I think life doesn’t need integrated intelligence to be life, though intelligence could probably be defined as a predictive kind of self-preservation process. Essentially devising a chain of actions for moving from state A to state B with a limited set of possible operations and minimized energy consumption. An efficient way to return to initial state, or to another state that increases likelyhood of identity preservation.

As someone else said in a comment, life may be seen as a continuum where these characteristics are more or less developed and integrated, for instance making coutries or viruses living organisms while still distinguishing the unique character of humans or animals.

[HPsquared]

As is typical for them, viruses are debatable under this definition (and how about computer viruses?)

[lawpoop]

> viruses are debatable under this definition

I think it's a thin pro-argument. Viruses are completely dependant on the host cell and the host cell's metabolism. The host cell's protein unwraps the jacket, and the host cells' proteins replicate the viral DNA/RNA payload. The virus does not reproduce itself, and it does not metabolize anything. There are no inputs to a virus.

Whereas living cells, give them the proper inputs, and they metabolize energy, catalyze reactions, and create copies of themselves.

[tachyonbeam]

I would definitely say biological viruses are a life form. They use as a substrate. However, you need the ability to mutate in order to evolve. Computer viruses can't do that at the moment, they are dumb machines/tools that just keep doing the same thing over and over until we wipe them out.

IMO, self-replication is the most fundamental characteristic, and the ability to mutate/evolve is key as well. The rest is all details.

[jimbokun]

"(and how about computer viruses?)"

They don't metabolize energy.

[patcon]

You might be interested in the little-known Chemoton model: http://web.archive.org/web/20210221212235/https://www.nation...

    1. metabolic cycle
    2. ~~stores information~~
    2. membrane/boundary
    3. template replication

More recent origin of life theories (even proto-cellular) focus on:

    1. periodicity/cycles
    2. compartmentalization/boundaries
    3. microdiversity/entropy

https://sci-hub.st/10.1016/j.cocis.2007.08.008

[PicassoCTs]

Life is self-replicating energy states. As thus, i would include stable energy phenomena.A https://en.wikipedia.org/wiki/Extraterrestrial_vortex could fulfill the requirements for life if it was able to manipulate its environment into producing a re-occurrence aka offspring. So self-replication, aka storing the instructions for re-occurrence into the environment system, is kind of in a flux in extreme cases.

[toddh]

#3 removes the whole class of AI based life forms that would not need to replicate. Our future AI citizens may not appreciate not being thought of as living.

[the_af]

That seems to me a form of begging the question: you assume "AI based life forms" will indeed be life forms, then argue the definition from the parent post fails to account for this new life form.

But that such a thing as AI life forms will exist and be accepted as such by scientific consensus is not a given, at all.

[guerrilla]

A bee hive and a country fit this definition. Is that intended? I'm not against it.

[jimbokun]

This seems like a higher order definition.

Bees and Humans fit the definition. Therefor, aggregations of bees and humans, like hives and countries, also fit the definition.

"Countries" seem more of a stretch, as replicating would technically mean creating other countries like itself. Not just sustaining itself into the future.

Do hives seed other hives? I suppose they would, so they better fit the definition.

[lawpoop]

Well EO Wilson posited certain eusocial insect colonies, including bees, as a super-organism. I think the idea has some currency in biology. The idea is that Darwinian selection is acting upon the colony, so it is the unit of evolution. Remember that eukaryotic cells are a symbosis of two prokaryotic cells.

Not sure how often or even if countries self-reproduce.

[selestify]

Are people who are sterile, and therefore incapable of #3, no longer alive?

[tw26411499]

Cells self-replicate. Cells are alive. People are composed of cells. Therefore, people are alive. Sterility is irrelevant.

[beaunative]

I'd think of those as shared properties rather than definition of lives