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I'm putting together a class on basic biology for kids and I will discuss living and non-living.

I'm looking for examples of things which are undefined in terms of living and non-living, if there are any.

People expect viruses to be alive but they are defined as non-living. Other examples of things that go against intuition are also welcome. Or things which were once classified one way and later redefined.

I couldn't find anything online.

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    $\begingroup$ Viruses are not undefined - they are seen as non-living. $\endgroup$ – Chris Jun 23 '20 at 9:48
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    $\begingroup$ @Chris: Most people who study viruses consider them to be living organisms. Many consider bacterial plasmids to be alive, along with other transmissable genetic elements. $\endgroup$ – MikeyC Jun 23 '20 at 16:29
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    $\begingroup$ Does this answer your question? Is there any definition of life which makes viruses undeterminable? $\endgroup$ – mgkrebbs Jun 23 '20 at 18:41
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    $\begingroup$ relevant xkcd: Alive or Not $\endgroup$ – acvill Jun 24 '20 at 21:51
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    $\begingroup$ @user1136 Haha :) $\endgroup$ – Kantura Jun 26 '20 at 16:23
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People expect viruses to be alive but they are defined as non-living.

This is actually one of the most widespread misconceptions I see. I think it's just one of those interesting but dubious factoids that spreads around in dinner party conversations. There certainly are some who take that position. But, after 11+ years of formal university education in biology, I can't think of a single professor or colleague who firmly embraced the position that viruses are non-living. Most (including myself) have been firmly in the camp that viruses are, in fact, alive. And the rest are more-or-less agnostic or ambivalent on the topic.

There's an intesting mini-debate between experts on the subject found here, I'll share this quote from one of the participants below because I think it's on point.

"In many ways whether viruses are living or non-living entities is a moot philosophical point."

There isn't one single, universally accepted definition of life. So ultimately, the answer to whether viruses are living or not will depend on ones chosen definition, and that's the case with most other controversial things.

However, viruses are unquestionably classified as biological replicators. There's a review on that topic (found here). It also includes discussions of other replicators which some experts also think of as living entities (but many don't). Examples include certain genetic elements like plasmid and integrons.

One notable exception that I didn't see mentioned in that review is prions. The first virology professor I ever had strongly advocated definitions of life that were inclusive of prions, but I haven't found that position quite as common among other academics. I'm a bit more agnostic on the prion debate, because it's hard to find prion-inclusive definitions of life that arean't also inclusive of something like an inorganic crystal.

The problem is that scientists like consistent definitions that don't have lots of exceptions, or require us to split existing categories. Between viruses, plasmids, and prions, there's a lot to talk about in terms of what characteristics they share with other life forms and what they lack that makes some question their classification. It's also interesting to consider whether any of the attributes common to living things occur in silico (or if they could in the future).

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  • $\begingroup$ It's kind of contradictory that you use dismissing language such as "widespread misconception" and "dubious factoid", but then cite a debate article because to whom is the paper targeted to if not to experts with opposing view points? And calling it a moot point because there is no universal agreement is counterproductive as there aren't many thoughts that are universally agreed upon. It's like saying studying phylogeny is pointless because my next door neighbour believes in creationism. $\endgroup$ – Cell Jun 26 '20 at 18:34
  • $\begingroup$ Also your post puts a lot of emphasis on replication although it's not a key necessary trait of living things. A yeast cell with a temperature sensitive allele of a telomere capping protein will arrest permanently in stationary phase at a restrictive temperature unable to divide. Humans can become sterile in their lifetime. Both of those are incapable of reproducing, yet are unambiguously living things. Conversely if the vegetative yeast cell or the human completely lack metabolic activity they are unambiguously dead. Metabolism is a more important criterion. $\endgroup$ – Cell Jun 26 '20 at 18:39
  • $\begingroup$ @Cell. It is a widespread misconception that viruses are non-living, not becuase it's false, but because it isn't settled fact (which is how it was framed in the question above). I never implied that sterile humans weren't alive, I merely referenced one classification encompassing viruses and other biological entities. I'd happily reply to your critiques point-by-point, but I don't think the comments are an appropriate place for it. If you find my answer factually inaccurate, please specify where and I'll try to fix it. If you find it incomplete, consider writing your own. $\endgroup$ – MikeyC Jun 26 '20 at 19:53

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