The most common argument against insects feeling pain used to be that they lacked the nociceptors (sensory neurons that respond to stimuli indicating damage) that provide pain perception in humans, and by analogy other vertebrates.

However, it turns out that fruit fly larvae have thermal nociceptors that respond at the same threshold (~42°C) as those in vertebrates, rather damaging that line of argument.

Alternatively, we could state that they don't have the mental capacity to experience pain as a conscious human might, but this is difficult (maybe impossible) to prove: how can we objectively measure what is essentially a private, subjective experience?

This also suggests that the conceptual line drawn between vertebrate animals and other living things regarding which species can experience pain, is really quite arbitrary.

My stance as a vegan is that it is best to err on the side of assuming they do experience some form of subjective reality that probably includes pain. Most insects possess ganglia that integrate and process sensory information, and I would argue that the very process of integrating this information implies that a subjective representation of reality is being created at some level. Pain is a fundamentally important signal for complex organisms; I would really be very surprised if insects did not have some way to detect noxious stimuli.

>the conceptual line drawn between vertebrate animals and other living things regarding which species can experience pain, is really quite arbitrary.

It absolutely is. Right there in the name it is clear that we are mostly divided due to the ease of identifying certain anatomical structures. The way we categorize more complex forms of life is flawed in that it doesn't properly reflect convergently evolved functions. With more 'simple' forms of life like bacteria we get it right in that we more often sort by functionality: methane producing, carbon fixing, anaerobic, aerobic, etc.

Plants are also known to detect and respond to noxious stimuli. Now what?

Good question! The impression I have is that we all, vegans included, draw a line somewhere, and that there's almost never any solid reasoning behind anyone's decision on how to circumscribe his diet. I'd be really interested to hear an avowed vegan explain why it's not okay to kill and eat animals which are capable of nociception, but okay to kill and eat plants which are also so capable.

Nociception implies that the raw sensory data is being processed or encoded in some way. This can't happen without a nervous system. Plants don't have a nervous system, ergo, nociception is not occurring. I am quite convinced that this is consistent with our current understanding of the biology, but would be happy to learn differently.

Another way to approach this ethical dilemma is to proceed from a position of harm minimisation. When you consider energy inputs required to generate plant based calories versus animal based calories, the former is just about always more efficient. To the extent that we consider the moral harm of animal suffering greater or equal to plant suffering, if it exists, we must tend to substitute to plant-based calories to minimise the total suffering we inflict on other life.

It's easy, you just draw your arbitrary moral line wherever your outrage stops.

That's a different model from the "pain stimulus" model.

And "drawing a line" still requires one to specify what metric space the dividing line is perpendicular to.

Well yeah, but since it's likely everything experiences pain in some way (even plants), the "pain stimulus" model is flawed from the get-go. What people who use the model actually do is just set some threshold on their personal outrage or disgust scale and don't eat anything over that.

Or sometimes they just set the bar to where they think they're supposed to even if they don't necessarily even feel anything in particular about the issue. It's just more socially acceptable to eat carrots instead of house cats.

I think you are conducting a lot of armchair psychoanalysis without any basis. Since I am conveniently present here to help test your theories about my moral views, why don't you take the opportunity and engage in good faith?

Let's start with this: Why is it you think that everything experiences pain in some way? Can you elaborate on what you think 'experiencing' or 'some way' entail?

How about this, can you guarantee that not every living thing experiences pain in some way? Is a vertebrate nervous (or descendent of bilateria) system the only mechanism or can all living things respond to threatening stressors according to their own way?

Or even more general, and more important than just pain, can you guarantee that every living thing doesn't "experience" the universe according to their own way? Perhaps in ways beyond our ken?

(I think the general experiential test is more important than pain, things that don't feel pain shouldn't be invalidated because of that. Otherwise people who have no pain would be valueless, which I think we can agree isn't right either).

If a living thing wasn't gifted with neurons and fast response muscle fibers then is it fair to draw the line such that "only things gifted in experiencing the universe like I do are off limits because those are the only valuable experiences worthy of life?"

For example, we know plants can "move" (twisting and turning of leaves, growing of branches and root) to get more sunlight and soil nutrients. And plants can respond to long-term and near term threats with a variety of short and long-term responses...even sacrificing themselves in some cases to cast off the literal seeds of the next generation if need be.

Some plants have even built carnivorous mechanisms so smart they can tell the difference between dietary animals in its diet and animals not in its diet -- most dogs can't even figure this out and will gobble down all manner of things. These plants have no recognizable nervous systems and we're not even quite sure how they experience the universe such that they can do this, and then move faster and stronger than their prey in order to eat. Can you promise me that plants don't feel something? Even in a way, and using mechanisms and cognition, different than our own?

We know plants can smell, see, remember, communicate with each other, can signal different parts of itself, have memories (all of these of course according to the ways of a plant) and other hallmarks of "higher-level" organisms. Can you promise me that among these things, a plant doesn't hurt when it's turned into an ingredient in a salad, or baked in an oven or sautéed.

Single cell organisms even show many of the fight or flight responses of larger, more, complex, more neurologically gifted organisms. They'll hunt out and find nutrients, flee from threats, some will create and release stress molecules when under attack. "It's just chemistry" just insults the unique nature of living things. You're just chemistry also. Just because your ancestors developed a more complex Rube Goldberg machine for threat response and finding nutrients doesn't mean you should diminish those living things less fortunate than you. Can you absolutely guarantee that microorganisms don't experience "pain" according to their own way?

We know different kinds of living things work in different kinds of ways. So different it's hard to even come up with a simple definition of "life". Is it any surprise that different living things have different kinds of experiences and ways of "computing" those experiences?

Plants have glutamate receptors (analogous to higher organism's system for memory formation and learning). Sea Sponges, just like plants, have no neurons at all, but can use various mechanisms for whole body contraction and other intra-cellular communication -- like calcium waves. Radiata, some of the simplest multi-cellular mobile animals, have nervous systems, but some threat responses, like stingers, are wholly automated and disconnected from their nervous systems entirely.

You don't know if a cow or a pig or a chicken experiences pain. They could just be operating according to an evolutionary pre-programmed instinctual chemical trigger response that you are simply personifying as pain. The reaction of a cow to a threat is not much different than a bacteria to a macrophage.

Let's suppose an Alien drops to Earth tomorrow. But because of how their species evolved experiences the universe in ways we can't even begin to imagine. Should we invalidate its existence and claim it as a possible food simple because of this?

Do you draw the line only at "things with nerves" or "things with an identifiable central nervous cluster" or "things that move on timescales I can recognize?" Or "things that have pain responses I can recognize?" or "things that have pain responses I can personify?"

Is your diet okay simply because we're ignorant of the alternate experiences of other kinds of life, or did you just draw the line there because that's where you stopped feeling bad?

Is ignorance and lack of imagination a valid way to draw moral thresholds? I don't know if a plant hurts when I eat it, therefore it's okay?

Do they integrate sensory information from those stimuli though? Do they create a subjective representation of what those stimuli are, and use that representation to guide their actions? I think those are the questions I would ask to find out if they could experience something as subjective as suffering. As far as I know the answer is negative for plants.

If you could describe something functionally akin to ganglia or a CNS in a plant, I would have to reconsider my position.

> how can we objectively measure what is essentially a private, subjective experience?

We can't measure "subjective experience", nor can we verify that it even exists; which suggests that it is not a well-defined concept and we shouldn't be using it for anything.

From a philosophical point of view you may well be correct. However, there is the option of using behavioural observations as a surrogate. For example, exposing living things to pain stimuli and analogising the response to what we humans know to be a typical reaction to pain.

This does carry the risk of inappropriately anthropomorphising non-human species, but as the question of whether other creatures feel pain is intimately linked to the morality of how humans should treat them (after all, we're not going to be able to stop other animals from inflicting pain on each other), then maybe it's not a problem.

I have a microcontroller, a tiny DC motor, a pair of wheels, and a handful of thermistors. From these I build a robot, autonomous to the limit of its battery charge, which senses the temperature of its surroundings and acts on this information to move itself toward temperatures within a range its programming defines as preferable. The speed with which it does so is determined, to the limit of the motor's capability, by the difference between the temperature it's moving away from, and the temperature it's moving toward.

If I use a cigarette lighter or a lit match to apply an open flame to one of this robot's thermistors, it will flinch away from the stimulus as fast as it can. This is closely analogous to "what we humans know to be a typical reaction to pain"; if you applied the same stimulus to one of my fingers, after all, I would initially react in much the same fashion.

From this behavioral observation, do you conclude that my robot's subjective experience a) exists at all, and b) encompasses the experience of pain? Does this behavioral observation inform your concept of the morality surrounding how this robot is treated by humans? Why, or why not?

You raise a good counterpoint. Perhaps it's less to do with analogy and more to do with empathy. Which is again a subjective appraisal of the topic.

... but your robot isn't delicious...

Not to us, because we don't eat rocks and metal. If there's a planet out there whose apex predator species is silicaceous in the same way we're carbonaceous, then, there, my robot is halfway to being a cricket, and is probably quite tasty when properly prepared.

(I eagerly invite speculation on what, on our notional silicaceous Earth-analog, might constitute a parallel to veganism.)

Only eating Apple. Drrump, tissch!

Next time you're in the DC/Baltimore area, look me up! In exchange for a laugh like that, the least I can do is buy you a beer.

Hmmm, next time you're in Bonnie Scotland you can buy me one ;-)

Many of the most important concepts in science, not to mention ethics, are not yet well defined... I don't think your stance here is very productive.

Where do you draw the line though? Ultimately it's arbitrary and I think vegans realize the only solution is to avoid the issue entirely.

Plants, as mentioned elsewhere in this thread, are also capable of nociception. In advance of further discussion, my preliminary conclusion is that the vegan line is just as arbitrary as any other, but drawn so as to more tightly circumscribe those species on which the infliction of pain is acceptable, presumably out of a desire to inflict the least possible harm -- an honorable intent, but I think not honorable enough to satisfy at least some vegans, who insist upon seeing themselves as taking the only moral option, rather than merely the least practicable immoral option.

(I have to admit I don't really understand any of this dietary hair-splitting from the inside. My species has gone to a lot of effort in becoming this planet's undisputed apex predator, and as far as I'm concerned, I may as well get the benefit.)

We aren't the apex predator. Yes, we do have everything necessary to kill any one animal on Earth and eat it. Even animals that are usually poisonous, like the pufferfish, the fishing of which is strictly regulated because otherwise we'll eat them all.

On the other hand, we're not actually eating all the species. Most of us are almost exclusively eating species that through the miracle of selective breeding have been bred to be delicious, nutritious and easy to kill.

We stopped trying to satisfy our dietary needs by way of becoming the apex predator once we figured out domestication.

"Apex predator" doesn't mean "eats everything", but rather "eats what it please and has no predators of its own". By that standard, we're the most spectacularly successful predatory species this planet has yet seen, to the extent that many of us never have to hunt even once in our entire lives.

Plants are not capable of nociception, as far as I can tell from my familiarity with the biology. They don't have nociceptors. They don't have integrating organs to interface with nociceptors. They don't feel.

I'd be happy to have you explain how this is not the case.