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Let's first define "living". A living thing is an object that shares some traits with other living things, e.g. it has a metabolism, it has been "born" (in the broadest sense, including cell division and so on) and it changes throughout it's lifetime and will, eventually, someday die.

Imagine you see a picture of a tiger: You instantly recognize it being a living thing, more specific: Something you call "tiger". You have seen tigers before (or at least heard of them and saw pictures or so). You have somekind of (vague) concept of what a tiger might be and this thing fits percectly in it. And you know that a tiger is a living thing and you believe that what you see is a tiger: So you believe that the thing you see is a living thing called tiger.

Nasobēm

Then, a bit later, you see a picture of a Nasobēm. You've never heard of that before (if you did, assume you didn't). You have no concept at all of what a Nasobēm is. You haven't heard anything from it and still you somehow see: It (might be) and it is very like to be a living object. Still, you haven't seen anything resembling something Nasobēm-like ever before in your life.

What exactly are the features we all recognize living things kind of acurate (even though when they don't exist in reality and are, just like the Nasobēm, just imaginary animals someone invented)?

Of course, these are not perfect algorithms, but just kind-of-okay-heuristics, meaning, their result might not be perfect, but it's good enough for "everyday-usage". But still: It has to rely on things that we ourselves observed and considered "living", but what phenomenons do we actually use? Remember, this is just a still-picture, you cannot see anything moving (which might be an indicator for living things in one way or another).

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    I think the relevant fields are psychology, neuropsychology and neurophysiology. E.g., here. With your comment below the current answer, I think I can safely say that this is off-topic here and may find a better home elsewhere. Perhaps Biology SE? – user3164 Nov 1 '13 at 17:10
  • The sentence "What exactly are the features we all recognize living things kind of acurate..." is confusing, it is not well-formed English, yet it seems to be the crux of your question. Can you rephrase it just to make sure we understand your question properly? Other than that, this topic is/can be on-topic here, as long as it is referring to how we should define life in general, rather than specifically how biologists currently do it. Under the former head, this question falls under Social Ethics, which — as a subcategory of philosophy — is surely welcome here. – stoicfury Nov 2 '13 at 8:49
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    My other question is, why do you only want visual cues which indicate life? I don't know of any definition of life which requires solely visual confirmation; we merely infer life through the visual but we wouldn't necessarily strictly define it with only visual cues. – stoicfury Nov 2 '13 at 9:01
  • Posted this on Psychology & Neuroscience ty for the inspiration cogsci.stackexchange.com/questions/4866/… – user19651 Nov 2 '13 at 11:51
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    This question appears to be off-topic because it is narrowly defined in terms of a single form of sensory perception, rather than say a general framework (however heuristic) for considering the classification of entities/phenomena as 'definitely life' or 'definitely not-life'. (The fact that the example fails to provide an intuition pump because it is an obvious re-mix of a rat, a squid, a jellyfish, and Bugs Bunny doesn't help.) – Niel de Beaudrap Nov 2 '13 at 18:27
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The question is definitely not philosophy, but whatever. I also don't think you're going to get a particularly interesting answer; we spend early childhood being shown a huge array of animals, and we get pretty good at telling when a new object is similar to an animal we've seen before. Most vertebrates have roughly similar structures (eyes, a head, limbs) and textures (fur, feathers, skin) which aren't similar to most non-animals. And there are very clear cases of our failure to recognise life (think of fungi or mould, which we have to be convinced is technically life).

As for your specific case, it's a squid grafted onto a mouse walking almost identically to how humans walk; I don't think you should be surprised we see it and assume it's a living thing.

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    I disagree entirely on the "not philosophy" claim. Here's why: we get to decide what to call life. That decision needs to make some sort of sense, such that 'life' is a natural kind. – labreuer Nov 2 '13 at 1:48
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    That's not the question, though; the question is what heuristics we use to identify life when we see it. The question of what life is is certainly philosophy. – dbmag9 Nov 3 '13 at 23:56
  • I suppose the questioner needs to ask a question of how the average person sees life, then. And yep, that's just cognitive science. I'll reverse my -1 if this is edited (SE requires an edit). – labreuer Nov 4 '13 at 0:37
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I think it's more about experience than anything else. Mushrooms don't move. Eggs may have different forms, colors and sizes. As biologists, we run into very strange structures in a forest that we have no clue if they're alive or not.

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  • That's correct. As I said, it is more like a heuristic than an algorithm. It's not perfect, but still mostly-okay. Whatever: Even with experience, we have to pick subliminally some features, that we sublimially choose to distinct living objects from non-living objects. My question is: What are those traits? – Perik Onti Nov 1 '13 at 17:26
  • They vary from one taxon to another. Sometimes the texture. Sometimes the softness. Sometimes some known structures (petals? feathers?) Anyway, where is the thread that defines "what is life"? – Rodrigo Nov 1 '13 at 22:11

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