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It seemed fairly basic at first, and yet I have to ask because even a quick Google search reveals that it doesn't seem to be quite so straight-forward anymore. After more research, I found multiple lists, some with six points, others seven or eight. Some lists shared six points and only differed on one, but others had five different points and divided and named the points differently such as Respiration vs Metabolism, Adapt vs Evolve, Homeostasis vs simply consisting of cells, etc.

Maybe it never really was simple, but when I learned basic middle school life science in the 1990s I seem to remember a clear and concise list of 7-8 distinct terms. I know they were just the summary given by the author of whatever textbook we used, but I remember several sources having similar lists (cannot say for sure if they were exactly the same), including a textbook, an educational video game, and a poster.

I'm also told that this has become a bit of a debate in the biology community, whether such a black and white distinction of "living" or "not living" is prudent. Questions such as this one on viruses are an example of how it is difficult to categorize life. However, I noted in my research that there was still clearly overlap in meaning of definitions, it was just in description and division of points that they differed. This suggests to me there is still a generally agreed upon core definition that is simply described by different people, specifically authors, with a few areas of possible debate.

  • What are the current generally agreed upon characteristics, the common denominators? (They all reproduce?)
  • What characteristics are commonly debated and therefore would unwarranted to claim? (Specifying cell structure?)

With the understanding that an answer will require an objective synthesis of a range of content and opinions, debate on what should or should not be characteristics is discouraged. Rather, an impassioned summary of the current scientific consensus on this question is what I am looking for.

It seems to me that even if this is a topic of opinionated debate, it is certainly at the core of Biology and would be worthwhile to be addressed in an academic manner, be accessible, and available to reference on this site.

(PS - Not in any way a homework question, never was, just a curious 30-year father. Unless you are counting questions I have to answer from a curious little boy.)

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  • $\begingroup$ I asked a related question. "Does it have it's own ribosomes?" is a good objective classification, but why should anyone care? Ultimately any answer is subjective. $\endgroup$ – James May 12 '16 at 6:46
  • $\begingroup$ The definition of any subject is open to subjectivity. Asking "What is maths?", "What is programming?", "What is beginners English?" or "What is religion?" in the respective SEs would all be equally be off topic. Additionally, I think the attitude of approaching a new community expecting their answers and then insulting the community is particularly unpleasant. $\endgroup$ – James May 13 '16 at 17:35
  • $\begingroup$ @James I agree...if that is what I had asked. But I am asking "What are agreed upon characteristics?" This can be done objectively by simply reviewing all the subjective definitions and summarizing what they agree upon. As you say, everything is subjective at some point, so that is why scientists have peer reviews and such, so by the use of consensus of experts we can arrive at a conclusion that can be labeled objectively correct. And I still believe that voting to close a question with no comment as to why, when it is not obvious, is far more insulting than anything you think I've said. $\endgroup$ – Joshua May 13 '16 at 17:46
  • $\begingroup$ @James if I am simply not communicating it well I will try to make an edit this evening. But it is a bit difficult to know what to correct with no constructive feedback (besides your own recent comment which did help) so if you or anyone else thinks they can see where I'm going wrong here I'm still just not sure, even after an edit, that it won't be closed again for some unknown reason. Maybe I've expressed my frustration poorly, but can you see that the attitude of approaching a new member of a community and expecting them to know what's wrong without telling them is equally unpleasant? $\endgroup$ – Joshua May 13 '16 at 17:54
  • $\begingroup$ I didn't mean to say everything is subjective. There are empirical evidences. The issue comes when trying to devise a test to answer "what is living?" Many philosophers have moved away from the living/non-living divide & prefer something a little more nuanced. You've unwittingly stumbled into an age-old, unoriginal, unanswerable question. Here, we like questions that are answerable & want to avoid questions that induce debates. Because of this I'm not sure if any amount of edits will help. Recently in the chat we even considered closing that virus question for the same reasons. $\endgroup$ – James May 14 '16 at 2:27
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First of all, you have to note that there is no such thing as a scientific, objective definition of life, and that many scientists disagree on a given definition, and that every definition has it's limits.

However, the one from wikipedia

Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:[10][12][13][14][15][16][17]

Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature

Organization: being structurally composed of one or more cells — the basic units of life

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.

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.

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.

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.

Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

Encyclopedia Britannica

  • Metabolism

  • Respiration

  • Sensitivity (responding to stimuli like gravity and sunlight)
  • Nutrition
  • Excretion (getting rid of CO2)
  • Growth
  • Reproduction
  • Homeostasis
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  • $\begingroup$ "you have to note that there is no such thing"... Part of this question is about dealing with just that. Generally agreed upon. Consensus. But you dismissed it and just gave me the results of a Google search (which I've already done). Was hoping for more. The latter encyclopedia version is what seems to me to have been a pretty standardized answer in the 90s. It was in textbooks, educational video games, etc. So if there was consensus then, why not anymore? $\endgroup$ – Joshua May 12 '16 at 10:52
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    $\begingroup$ @Joshua "textbooks" are not consensus. They are the opinions of the publisher. Anybody can write one, and if it's cheap enough schools will buy them. Textbook authors generally take the viewpoint that younger readers can't handle ambiguity, and so create facts where none actually exist. I'm willing to bet if you traveled back to your childhood, all the media you mentioned didn't completely agree. $\endgroup$ – MattDMo May 12 '16 at 12:04
  • $\begingroup$ @MattDMo if you combined that with a summary of what various definitions do agree on and what they don't, you'd have a winning answer. $\endgroup$ – Joshua May 12 '16 at 13:43
  • $\begingroup$ Just wanted to alert you that the question has been edited considerably. I believe its still the core question I intended, but hopefully I'm more clear this time. Thanks. $\endgroup$ – Joshua May 14 '16 at 4:27

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