Yes, the ability to feel pain seems to an important attribute of a “being” in the suttas. Though I’d broaden this to the ability to suffer.
Hmm, I don’t know where you would draw the line if the question is its ability to suffer.
Does an ant suffer when hurt? what does it generally mean to suffer? An ants life is made harder if it loses a leg or claw, but if it doesn’t care because it can’t think conceptually then does that excuse harming ants because they don’t suffer? I’ve seen crabs remove their own claws for no reason.
That’s why I think just sticking to its ability to physically feel (has a nervous system) is sufficient enough, rather than trying to determine if it actually suffers, which is a rather ambiguous and difficult thing to figure out.
I think that the idea that you need a nervous system to ‘feel’ is old science now. We have new understandings around areas such as chemotaxis in ecoli, and how bacteria form biofilms using touch, as a couple of examples.
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Sure, physical pain implies some mechanism to register the pain, usually pain receptors and a nervous system. Though I don’t think physical pain requires a mind, or concepts, or whatever, since organisms can respond instinctively. Thinking of the Arrow Sutta, there can just be the first arrow, ie bodily pain.
Oke, but can that bodily pain be without a mind?
I would say yes. Even simple organisms respond to unpleasant stimuli.
So a mind is required to experience the second arrow (mental anguish), but not to experience the first arrow (bodily or physical pain).
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Hi @Martin,
Sharing what i have learned: I have learned that the painful bodily feeling or sensation is a mental quality, a dukkha vedana. It is part of the tactile vinnana or body-vinnana. It cannot be separated from it. Vedana, sanna and vinnana are connected (MN43)
“Feeling, perception, and consciousness—these things are mixed, not separate. And you can never completely dissect them so as to describe the difference between them. For you perceive what you feel, and you cognize what you perceive. That’s why these things are mixed, not separate. And you can never completely dissect them so as to describe the difference between them.” (MN43)
What i have learned is that a bodily feeling is not a rupa. It is a vedana. It is also not felt by the body. Although it is called bodily (because it is related to some affliction of the body ) the painful feeling (vedana) itself accompanies the tactile vinnana.
So I have learned it also needs mind to experience the first arrow.
Based on that painful feeling or sensation, an emotion like aversion might arise. That vinnana, with that emotion is also accomponied with a kind of painful vedana, in this case domanassa vedana. It has a dark quality. This is the second arrow. It is because of patigha-anusaya (in this case).
But one can still question if some organism must have vinnana to show reactions on stimuli. The characteristic of vedana is; it feels, and of vinnana is, it cognizes.
"It’s called consciousness because it cognizes. And what does it cognize? It cognizes ‘pleasure’ and ‘pain’ and ‘neutral’. It’s called consciousness because it cognizes.”
“It’s called feeling because it feels. And what does it feel? It feels pleasure, pain, and neutral. It’s called feeling because it feels.” (MN43)
Plants grow to light, does that mean that they have a kind of sanna, vedana or vinnana?
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Usually the criteria is death of which consciousness is assumed. The death of a sentient being is being part of a food chain that distinguishes it from inanimate objects. Feeling is a basis for moral obligation, but the degree of sentience is determined through autonomy and free will.
Heedfulness is the path to the Deathless. Heedlessness is the path to death. The heedful die not. The heedless are as if dead already.
I’m not sure. The aggregates model doesn’t seem to work so well with simpler organisms. Maybe you could say a plant “perceives” light, and that perception is “pleasant”, something the plant is drawn towards.
But then a plant doesn’t have the ability to regard itself as a being, or as a self. Perhaps it’s this ability which indicates sentience in the suttas?
I’m trying to relate this to our other discussion about whether beings really exist.
The difference IMO is nana versus vinnana.
Nana, ie simple knowing occurs in single faculty living organisms such as plants or very simple animals, and is what lies behind their reactions to environmental stimuli.
Vi-nana (literally double knowing) is knowing that one knows. This is quite crucial as Sentience depends on it. Thus only those beings which have Vinnana - they know that they know and can recognize pleasurable, painful or neutral stimuli - can Suffer.
But at what stage do living organisms turn from having simple nana to vinnana? This, IMO is the cause of the scriptural ambivalence towards creatures such as Insects and worms (and in our modern day - bacteria, maybe viruses too??), which of course, deserve the benefit of the doubt for the sake of compassion.
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I guess it’s technically, ‘they know that they knew’ rather than ‘they know that they know’, streams of consciousnesses being what they are. But that’s just an aside, I’m interested in how you would assess the potential for e-coli to have Vinnana in this experiment?
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Knowing, nana, without vinnana feels, at first, a little bit strange, but i think it is possible. There is also a kind of knowing in us, i belief, which stays unaware but this we are informed.
I think we are informed about the external and internal world in many ways, and just a small part of this information becomes aware to us.
I still no not know what is sentient, but with years i have began to treath more and more beings as sentient, with feeling, seeking happiness, not suffering, in their own way. 20 years ago i still fished. I did not see the worm and fish really as unique living beings who also want a nice life, not suffering, happiness. I was not sensitive enough that time. I stopped this cruel practice. I committed myself to not killing and hurting. Very difficult. Life becomes more complicated but i felt it was good to do. Now it feels less complicated.
I do not know for sure an insect or a worm feels, but i treat them like that. Years ago it was very controversial to declare a fish felt pain, but now, here in my country, it is has become more and more accepted. Still there are many parents who teach their children fishing, i see.
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Sentient = capable of self-view, capable of thinking “me” and “mine”? And therefore liable to experience suffering? 
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Martin seems to me sentient 
Arahants are not capable of self-view, capable of thinking “me” and “mine” but still experience suffering. Indeed, that’s all that is experienced by Arahants.
SN12.15:
But if—when it comes to this attraction, grasping, mental fixation, insistence, and underlying tendency—you don’t get attracted, grasp, and commit to the notion ‘my self’, you’ll have no doubt or uncertainty that what arises is just suffering arising, and what ceases is just suffering ceasing. Your knowledge about this is independent of others.This is how right view is defined.
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…but still, when the Buddha thought about going to teach Dhamma he thought: "If I were to teach the Dhamma, others would not understand me, and that would be wearying and troublesome for me.’ (MN26)
If suffering is all arahants experience, why do we even talk about the cessation of suffering? It seems the goal of the Dhamma is not to end suffering but to suffer all the time??
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I think suffering has ceased for the Arahant. Second and third Noble Truths.
That is exactly my point!
For more complex animals such as cats and dogs, it seems to be easily observable. They remember things, they are attached to ‘their’ toys, places, people, they have moods and seem to definitely experience dukkha.
But it doesn’t seem as easy to observe in lower animals. The further down the food chain one goes the more difficult it becomes.
And we meet an added complexity - the difficulty of ascertaining if what seems to be vinnana is actually that and not an artefact. For example can an ant differentiate its nest from another or is it just a chemotactic response?
And then there are the endless possibilities of AI… could a self driving car which seems to remember, plan and respond develop a sense of Self? Even a sense of Self it seems, can be programmed.
So I guess, I sympathize with our early buddhists who decided to give the benefit of the doubt to the apparently less developed creatures, out of compassion. BUT, I feel that going to the other extreme like the Jains do would be a bit over the top too. 
Sure, post parinibbana it has ceased, but there is a lag between cause and effect, just like (say) the lag between birth and death in DO. The first dart is still there for the Arahant pre parinibbana, but the second dart has ceased. The first dart ceases at parinibbana because there is no more rebirth. At least that is my understanding.
According to MN121, the Arahant experiences only a modicum of stress associated with the sense-bases.