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Recent research has revealed that plants are able to communicate, learn, and respond to harm. It has been argued that these behaviours constitute basic intelligence. Robert A. Freitas Jr. has described plants as having hormonal sentience.

Are these intelligent behaviours sufficient for moral consideration in an ethical framework which seeks to minimize suffering?

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    I attempted to frame a meta question that is related to this, but also about broader questions of where philosophy and subjectivity intersect (or not). Please feel free to continue discussing and/or voting on closure, but sharing your perspectives in meta will help both this question and future ones. Thanks! – Erica Mar 15 '17 at 16:16
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    Cleaned up comments. Please keep things topical, use Veganism & Vegetarianism Chat for extended discussions, or write an answer if you've got a response to the question. – Erica Apr 5 '17 at 15:10
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The 'intelligent' behaviour that plants exhibit is not evidence for the capacity to suffer. Very simple organisms can show some kind of intelligence in their behaviour, and some think plants might even be conscious, but suffering is an entirely different matter.

From an interview with a scientist:

So, if I follow you, plants really do feel, not metaphorically, but really. They just can't feel pain. Right?

Plants don't have pain receptors. Plants have pressure receptors that allow them to know when they're being touched or moved—mechanoreceptors. It's a specific nerve cell.

And to be clear, am I right that a plant knows it's being damaged?

You can definitely kill a plant, but it doesn't care.

This article describes the work of a group of scientists pioneering the field of 'plant neurobiology'. At least some of them argue that plants are conscious in some sense. The article also discusses the heavy backlash that they get from most other scientists. And yet even in this group:

No one I spoke to in the loose, interdisciplinary group of scientists working on plant intelligence claims that plants have telekinetic powers or feel emotions.

Note the definition of pain includes "An unpleasant sensory and emotional experience..."

In short, the dominant scientific attitude is that there is insufficient evidence that plants can feel emotion, pain, or suffering.

When a plant's leaves are being chewed on by a caterpillar, it may release a chemical which in one way or another causes the caterpillar to stop (e.g. by attracting birds or making the leaves taste bitter). This is a simple mechanical/chemical response that doesn't require any level of consciousness, in the same way that your skin releasing melanin (causing tanning) in response to UV rays doesn't indicate that your skin is conscious.

If the plant found the chewing painful, then for this pain to be useful to the plant's survival, it would have to:

  • remember the unpleasantness afterwards.
  • be motivated to make decisions (e.g. which direction to grow in) in order to avoid a similar experience.
  • actually make a decision which resulted in less chewing by caterpillars than could be accomplished by the simple chemical release.

All of which would be quite extraordinary.

It's possible that plants feel pain and we haven't discovered the evidence and the mechanism yet. Just that chance is enough to say that yes, plants deserve moral consideration. The expected value of hurting a plant is negative, even if the most likely scenario is that it is morally neutral. But the absolute value is small, so I can't think of any situation where this would tip the scales of a decision. There are much more significant factors in deciding whether or not to hurt plants, such as the impact on the environment, or our nutrition. Plant pain is hypothetical, while there are very real risks of becoming unhealthy or unhappy on a fruitarian diet.

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    What do you mean by " while there are very real risks of becoming unhealthy or unhappy on a fruitarian diet."? – user1993 Feb 6 '18 at 2:15
  • @user1993 it's very restrictive, and as far as I can tell (not debating it here though) there isn't a consensus among professionals that it's safe and nutritionally adequate the way there is for veganism. – Alex Hall Feb 6 '18 at 8:05
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Preamble: I think this is a valid and on-topic question for this forum. This type of question can seem inflammatory for the reason that the asker doesn't realise they are entering such well-trodden ground. I hope we can maintain a community that is friendly and welcoming enough to patiently answer such questions.


TL;DR Yes if they have some degree of sentience, then they deserve moral consideration. Practically, though, they're the food source that causes the least amount of suffering.

Extended TL;DR: Yes, if there is good evidence of conscious experience, then of course they deserve moral consideration. More practically, if plants do hold moral value, it's definitely not a good idea to eat animals since those animals had to eat a large mass of plant matter to produce a small mass of consumable meat. For example, for every kilo of beef you eat, you're "eating" about 6 kilos of grain. If you ate the grain directly you'd get more energy, more protein, more fibre, more everything (except for a few nutrients like B12). You'd cause far less suffering by eating plants.


Answer:

If good science does show that plants are capable of conscious experience and suffering, then yes, they most certainly do deserve moral consideration.

Aside: Debating what "conscious experience and suffering" actually means could be within the scope of this question, but I think we can provide a practical/prescriptive answer without getting into that.

First things first: We must agree that consciousness and sentience aren't binary. Instead, varying degrees of consciousness are possible. Here's a terrible diagram:

relative moral value

Obviously this isn't anywhere near accurate or to scale or in proportion - it's just to illustrate that there is a spectrum of moral value. If we care about the moral value of other organisms, but we also want to survive (debates about selfishness aside), then we should try to ensure that our diets have the smallest practical negative moral consequences.

If you disagree that plants have a lower moral value than animals (like cows, chickens, etc.), then I'd have to call your bluff. I can't imagine that anyone actually believes this. If they'd just as easily burn a dog alive as they would a lettuce plant, then I think my convincing them is outside of the scope of this question.

So then the question becomes: How do we achieve the "smallest practical negative moral consequences"? What is considered "practical" by a person depends on their willingness to forego pleasure for the sake of reducing the harm they're doing to others. It also depends on the scientific consensus regarding the sentience of the various food candidates. It's still early days in our scientific understanding of consciousness and sentience.

Moving beyond these questions into more concrete prescriptions of good behaviour requires a lot of utilitarian and scientific ground-work and is probably best answered on a situation-by-situation basis. As an example of the "moral calculus" that we must deal with as we start to move into this territory: A person may justify eating plants by demonstrating that the effort that it would take to switch to and maintain a algae-based diet would detract from their other positive moral endeavours such that they'd thus be able to do less good in the world. Indeed, it may be quite hard to fulfil one's dietary requirements using only algae today, given that they currently don't exist in supermarkets on a large scale. One hundred years ago, this may have been a sane and valid argument for avoiding a vegan diet (pre algal B12 synthesis, for example).

Achieving the "smallest practical negative moral consequences" is a tough one, but since this is a vegetarian/vegan forum, the question is a usually little more simple: "Should we avoid eating animals?" In this case we can prescribe moral behaviour using only the knowledge that plants have a lower moral value than that of the herbivorous animals that we eat. The argument is simple: The animals that we eat must eat several kilos of plants to produce some smaller mass of food (meat, eggs, milk, etc.). The resulting animal product has less energy, less protein, less fibre and, in general, less nutrients than the original plant matter. Thus, if don't filter our food through the herbivores, we're able to get the same amount of energy, protein, etc. and have killed less plants overall. So we can see that there's no need to know the absolute moral value of plants if we just care about practical ethics.

So, in summary: Yes, plants do deserve moral consideration. We should try to avoid assigning moral value purely on the basis of species membership (or kingdom membership in this case). An organism's moral value should depend on its capacity for conscious experience and sentience as estimated via the available scientific data. Following this understanding, we should be careful not to fall into this trap: "I'd still be causing suffering even if I only ate plants. I can't stop all suffering, so I'll just give up altogether and not worry about morality." That's an irrational and immoral position to take. Instead we should attempt to minimise the suffering that is caused by our diets.

Aside: It's also worth noting that many of the plant products that we eat don't require us to kill or harm the plant. Fruits, nuts and many cereals/grains have evolved to work in symbiosis with animals such that the animals get to eat, and the plants get their seeds spread around (and often get a nice head-start be being embedded in nutrient-rich animal dung). Humans are in a species-level symbiotic relationship with the domesticated animals that we consume/use, but unfortunately the individual's well-being isn't increased by this species-level relationship. For example, bile bear farms may increase the number of sun bears in existence, and in that case sun bears would be in a species-level symbiotic relationship with the humans that breed them, but of course the wellness of the individual sun bears is not increased (rather, it is decreased considerably). So we've got to be careful not to conflate the "success" of the species with the wellness of the individual. In this way, the word "symbiosis" can be deceiving (certainly in its colloquial sense).


P.S. The OP's question was simply about whether plants deserve moral consideration. I've answered a broader question surrounding the "plant suffering" topic because it's hard to wrest the question itself from its common (often fallacious) implications.

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    Some might be interested in a more restricted form of veganism that causes as little suffering to plants as possible, e.g. fruitarianism. Not that I'm suggesting we do that, I personally don't believe plants can suffer, but your answer suggests that there is only a decision between veganism, non-veganism, and suicide. – Alex Hall Mar 18 '17 at 14:40
  • "If you disagree that plants have a lower moral value than animals..." what is moral value? Are you equating it with consciousness? Maybe my unwillingness to burn the dog relates to my ability to have compassion for her, or to witness evidence that she experiences suffering? – Zanna Mar 19 '17 at 22:14
  • @zanna This particular question considers minimizing suffering. So moral value would relate to ability to suffer. This could be made more explicit in the answer. – nloewen Mar 20 '17 at 1:36
  • "I can't imagine that anyone actually believes [that plants have a lower moral value than animals]" There are many people who will kill insects on sight but who will take care of a garden. Regardless of whether this is right or wrong, this creates a logical weakness in your argument. Dogs are not the only example of animals there is, you know. – Fatalize Mar 23 '17 at 10:27
  • @Fatalize People deciding to kill insects in their garden is not evidence that insects have a lower moral value than plants. I have no idea what leap of logic you're trying to make there. As I said in my conclusion: "We should try to avoid assigning moral value purely on the basis of species membership (or kingdom membership in this case). An organism's moral value should depend on its capacity for conscious experience and sentience as estimated via the available scientific data." – user116 Mar 23 '17 at 10:56
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Plants don't have feelings as we know them

So a scientist infests one plant with a bug, and finds out that another, non-infested plant has "communicated" with the first plant and starts bracing itself? And other plants can even learn? Therefore plants talk and think, therefore they are sentient? Should we stop eating them?

Well, watch this. When I send you an email, it arrives on your computer. Better even, my smart phone updates itself without me asking it. Google's DeepMind actually learns how to play Go. Some clever robots can even "recognise" themselves in a mirror. (Huge inverted commas here.) Wow, these gadgets communicated and thought! They must be sentient! Is it now unethical to wipe a phone's memory? (Don't say "Well, I bought it, it's mine, therefore I can do whatever I want", that would apply to animals as well).

What I want to illustrate is that communication and the ability to learn aren't indicators for feelings and consciousness the way we know. The reason I abstractly believe that other humans have a consciousness and experience pain the way I do is not because they have a face and talk to me, but because I know that they have the same kind of brain as I do. (Of course I look at their faces and listen to what they say in order to find out what they feel, but that's a different matter).

So my central point is:

Plants don't have a central nervous system, therefore they don't have feelings the way we do.

If you cut certain types of plant, they surely send out signals to the environment that someone is attacking them, and they change their hormonal levels and whatnot. But it's not pain the way you're feeling pain.

"But maybe the have some other kind of feelings?"

What do you mean by that? Your only way of really, certainly knowing about the existence of feelings is because you have feelings yourself. Other people certainly have feelings, but you can only know this by implicitly comparing their feelings to your own. Feelings, consciousness and thought are highly subjective experiences, and you can't expect vastly different beings or objects to have the same experience.

It is idle to argue that plants might have "feelings on their own".

"But why not eat animals, then? They surely don't have feelings either, then?"

Of course they have feelings. Look at the brains of vertebrates, they have the brain parts for experiencing pain and other feelings.

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    You argue that plants don't have feelings as we know them, and then conclude with animals have feelings, leaving the as we know them part. I don't think anyone would argue that any other animal has the same exact feeling as we human know them, therefore your argument is logically incorrect. Moreover, you say "Look at the brains of vertebrates", but vertebrates are only a small part of the animal kingdom. You say nothing about e.g. the feelings of crickets. – Fatalize Mar 23 '17 at 10:23
  • Turion: "It is idle to argue that plants might have 'feelings on their own'" - this statement doesn't mean anything. If you mean we shouldn't study other organisms like plants to determine their capacity for conscious experience then I strongly disagree. I think your general thesis is "communication and the ability to learn doesn't imply the ability to feel" which it certainly true, but you can't go on to claim that plant have a moral value of zero and imply that this is a certainty. – user116 Mar 23 '17 at 11:09
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    On the other hand, we can say that there's a very high probability that many non-human animals are conscious, and a low probability that known plant species are conscious. These are claims supported by the current scientific literature. As I explain in my answer, we only need this relative measurement to make practical ethical decisions regarding our diet. – user116 Mar 23 '17 at 11:12
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    @Fatalize, no animal has the same exact feelings as I have, nor does any other human. But I would argue that vertebrates can have feelings that strongly resemble a lot of feelings I commonly experience. Your point about the cricket is of course a good one. Where is the exact line to draw between "has comparable feelings to mine" and "doesn't have comparable feeling to mine"? I don't know. But I would argue that vertebrates belong to the former and plants to the latter. – Turion Mar 23 '17 at 17:27
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    @JoeRocc, I didn't want to imply that we shouldn't study whether plants, funghi, bacteria, computers, aliens, or whatever can be conscious. I actually agree with you that in some cases this is an extremely interesting question. The conclusion whether plants have moral value or not is left out in my answer. I think your answer already covers that very well. – Turion Mar 23 '17 at 17:34
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In short, Yes. But there is lot more to it than just that.

To weaken your argument, nervous systems of plants are far inferior to animmals. As many have pointed out, this severely affects the way plants feel or suffer, as compared to animals (See this https://en.wikipedia.org/wiki/Plant_perception_(physiology))

To generalise your argument, all physical systems or non-living beings exhibit some intelligence so that they prefer stability (H2 + O into H2O) but resist harm or changes which destablises their present state ( H2O into H2 + O).

Therefore, by extension, any form of destruction of nature, living or non-living, calls for moral policing. But then, how do we choose what to consume and what not to? This question, known as Dharma in Hinduism, can be answered both philosophically and scientifically, and the answer would be incomplete without attending to it.

Food is essential to sustain our lives, and therefore, our lifestyle becomes most important factor to make that selection. What we do, where we live, our culture and philosophies, temporal contexts (like famines!), are few of such factors. Therefore, plants have nothing specific to score over animals here.

So, why choose plants over animals? I'd ask: why not non living things over plants? or even nothing over something? This question is answered by the simple logic of "minimize destruction" of the present state, as in physics. Consuming nothing is better than consuming something, consuming non living things is better than consuming plants, consuming plants is better than consuming animals, consuming animals is better than consuming humans, and so on. This is because latter exists/survives on top of former, hence consumption of latter will certainly lead to more rapid consumption of resources than former, and therefore faster destruction. We'd reach faster to the stage when our consumption is more than production, by consuming latter than former.

To sum it up, choosing plants over animals has lot more to do with stability of our system (global warming, health issues etc) than any other factor (animal cruelty, which is more intelligent etc).

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Perhaps for moral satisfaction towards the idea, have your own plants to eat/look after - therefore limiting possible suffering on a global scale and continuing a cycle of planting and replanting.

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All living organisms are massively complex machines, they execute very elaborate algorithms to maintain homeostasis, to appreciate this, one can look at modern ideas on the origin of life, e.g. this article or this talk (first part and the second part. One may then argue, on the basis that intelligence and computation are related, that even primitive organisms have at least some limited degree of consciousness and even intelligence.

The question we can then ask is if we really cause a plant to suffer if we kill it, assuming that it is actually capable of feeling pain. This is not trivial due to a localization problem that also crops up in a totally different issue. The problem here is that any system that processes information will only have a finite amount of information and on the basis of that information, it cannot tell in which environment it exactly is in. If we then assume that alternative possibilities really exist (e.g. in multiverse theories such as the many-worlds interpretation of quantum mechanics), then each alternative possibility is just as real.

The point of view of a simple algorithm executed by a plant is then not detailed enough to localize it precisely in your garden where you are about to cut it. Exactly the same algorithm processing exactly the same data from its environment would be running in many, many different environments, e.g. one where it exists on a planet like Earth in a tropical rain forest. In some of these cases the next data it will process will cause suffering due to it being eaten or cut to pieces. But these are unlikely continuations, just like you being awoken due to an asteroid impact and then ending up starving to death in the coming months. In the multiverse this actually happens every day you wake up, but because you have many, many more exact copies in sectors where this doesn't happen, you are unlikely to ever experience this.

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