Premise(1) : If a sentient being can consciously experience pain and suffering, then it is wrong to inflict pain & suffering on such a sentient being.
Premise(2) : Plants are sentient beings that can experience pain & suffering.
Conclusion : It is wrong to inflict pain & suffering on plants.
In order to challenge the acceptability of premise(1), the anti- AR would have us believe that such a premise ineluctably leads to the absurd conclusion as stated above. In order to achieve this coup de grace of animal rights, the anti-AR who would give little or no coin to premise (1), would instead introduce the claims of premise(2) as somehow "scientifically established". In order to debunk animal rights as foolish, the anti-AR would first have us believe in the reality of "plant pain". Hence, they would attempt to bury AR into a hole but ironically by first bulldozing a much deeper one for themselves.
Someone like Rich Young would bait us with the following argument, an argument that presumably he still holds as having merit by virtue of his repeated postings of this worn polemic:
AR: "You're crude and unfeeling; you'd probably laugh at your mother's
non-AR: "That's silly, my mother is a human. A deer isn't."
AR: "Deer can suffer, and so do cattle...so I don't eat meat."
non-AR: "You apparently have no problem killing plants, though."
AR: "It's not the same. Plants aren't animals."
non-AR: "You're killing a living thing for food, nevertheless."
AR: "But it can't feel; it's not sentient; it has no nervous system."
non-AR: "Does dissimilarity rule out 'pain'?"
non-AR: "That's completely illogical and unscientific."
Note how Rich would invoke the authority of logic and science as "completely" on his side. Next, consider the assertions of Toby Bradshaw:
As a plant molecular biologist with quite a few refereed papers on the subject of cellular communication in plants, please allow me to debunk the unsubstantiated mythology described above. Plants have no *need* to feel pain? Ridiculous.Here we have the authority of logic, science and "truth" being imprecated against the sorry state of AR nescience and "mythology". Yet, no single published book, or paper in a scientific journal, has been cited as indeed making this claim that "plants feel pain". Sure, there is interesting evidence about plants reacting to local tissue damage and even sending signalling molecules serving to stimulate certain chemical defenses of nearby plants. But what has this got to do with supporting the only morally relevant claim worth considering, namely that "plants FEEL AND SUFFER from pain"? Where are the scientific references for this putative fact?
When a plant is attacked by an herbivorous insect, might it not be in the best interest of the plant to mobilize its chemical defenses in other parts of the plant in anticipation of further insect attack? When a leaf is infected by a pathogenic fungus, might the rest of the plant wish to bolster its chemical and enzymatic defenses against the spread of the pathogen? News flash -- the plant *would* benefit, hence the development of a systemic (throughout the plant) response to local tissue damage by herbivores and pathogens. (Many) references available upon request. It might easily be argued that *because* plants can't move they need effective chemical defenses and effective detection and communication. This is the case. You may doubt the sensory and integrative abilities of plants, so I invite you to spend a few weeks in my lab and learn the truth. Plants don't have nerves, since they don't share a particularly recent common ancestor with animals. Plants feel tissue injury and respond quickly, precisely, and with an effective battery of defenses. They don't feel *like us*, but it would be a mistake to say that they *don't feel*.
Now, dear reader, please be patient with my indulgence to develop a reasoned reply to such assertive and authoritative pronouncements about plant pain.
Furthermore, rain clouds bear meaningful information about their level of oversaturation in the form of weight relative to volume. Do not clouds, therefore, "sense" (in some tortured notion of the word) when atmospheric pressure is insufficient for their moisture content to remain in a vaporous state?
The promoters of plant pain would have us believe, against our good common sense, that by the mere presence of purposive BEHAVIOURS of avoidance and REACTIONS to tissue damage in plants we therefore must attribute to plants mental states like that of some kind of "felt pain".
Well, then by the same logic we must do the same to clouds. In the hole that these promoters of plant pain would dig for themselves, not only must we accept the thesis of plant pain, we would also have to swallow some notion of "cloud sentience"!
Let us further observe that the attribution of morally relevant mental states to even humans was at one time an issue of contention. For example, consider the case of that very prestigious scientific apologist of his society's ambient prejudices, Silas Mitchell, founder of American neurology. He claimed that civilized men suffered pain in a far more ethically relevant manner:
"In our process of being civilized we have won . . .intensified capacity to suffer. The savage does not feel pain as we do" .Today, we can witness a similar prejudice that animals do not suffer pain to the same capacity as we do. For instance, a cow after surgery will right away start eating grass, therefore it will be said that the cow cannot be suffering from post-surgery pain. Just as with the stoic "savage", who is to say that a cow is not likewise simply bearing the pain more "heroically" since, as with the non-civilized human, food is more of an imperative than moaning with pain; indeed, what else can they do?
So then, how do we properly attribute the existence of mental states to other animals, or even to ourselves for that matter, since in the past we have certainly made mistakes on this score? As we have seen, the *criterion of outward functional behaviour* has been faulty with even humans. Yet, our plant pain promoters would employ this same criterion at a different level, turn things on their head and argue that because plants react to noxious stimuli, they therefore feel pain. Now, if the inference of pain from overt behaviours has been faulty for attributing pain where we now know pain most assuredly exists, then it is probably equally faulty in attributing pain where pain does not exist. If reactions or behaviours were sufficient, then we would have to say that a mere toy doll crying and wriggling, when triggered to do so by certain stimuli, was indeed in pain.
Similarly, we cannot infer the presence of felt pain simply by the presence of a sub-class of behaviours which are functional for an organism's amelioration or avoidance of noxious stimuli. Thermostats obviously react to thermal changes in the environment and respond in a functionally appropriate manner to restore an initial "preferred" state thereby maintaining an equilibrium of the status quo. We would be dirt foolish, however, to then attribute to thermostats that therefore they must "sense" or "feel" some kind of "pain". Even warning quotes around our terms don't protect us from such an catachrestic absurdity.
Clearly, the behavioral criterion of even functional avoidance/defense reactions, is simply not sufficient nor even necessary for the proper attribution of pain as a felt mental state. This is not to say that it is completely irrelevant for it can at least index the presence of pain in those creatures we already know or have good reason to believe experience and suffer pain. Behaviour by itself does not index pain in our toy doll or thermostat, but behaviour does usefully index the occurrence of pain and suffering in those animals that we already have reason to believe have the capacity to suffer.
Let it be said at the outset that scientific materialism as such does not preclude the existence of emergent or functional qualities like that of mind, consciousness, and feeling (or even, dare I say it, free will), but all such qualities are dependant upon the existence of organized matter. If there is no hardware, there is nothing for the software to run on. If there is no intact, living brain, there is simply no mind. Now, just for the record it should also be said that even contemporary versions of dualism or mind-stuff theories will also make depended their embodied mental states in this world on the presence of sufficiently organized matter.
To briefly state the case, what is referred to as non-reductive materialism  would simply consider cognitive functions like consciousness and mind as emergent properties of sufficiently organized matter. Just as breathing is a function of a complex system of organs referred to aggregately as the respiratory system, so too is consciousness a function of the immensely complex information-processing capabilities of a central nervous system. Now, according to such a neo-functionalist account of mental states, HOW the matter is organized and in with WHAT materials is not necessarily delimited to the mammalian brain. It is possible in theory, that our Alpha Centaurians who evolved from carrots could equally instantiate some "higher" functions of consciousness. This may even be possible with a future computer given a sufficiently complex and orderly organization of its hardware and clever software. While such a computer does not yet exist, and we don't yet know about those Alpha Centaurians, we DO know that certain living organisms on this planet do possess the requisite complexity of specialized and highly organized structure for the emergence of mental states.
In theory, plants could possess a mental state like pain, but IF, AND ONLY IF there is a requisite complexity of organized plant tissue which could serve to INSTANTIATE the kinds of complex information processing that is prerequisite to such higher order mental states as that of consciousness and felt pain. A mammalian brain is not necessary but an immensely complex hierarchically organized central processor of some form would be.
Now, where is the morphological evidence that such a complexity of tissue in plants exist? Single cells or even aggregates of surrounding tissue is not sufficient for there to be a functional state of felt pain any more than even todays complex integrated circuit chips evince consciousness of any kind. A lot is required and plants just don't have it. This is not to say that they cannot exhibit complex reactions, but we are simply OVER- INTERPRETING such reactions when they are designated as "felt pain".
With respect to all mammals, birds, and reptiles, we know that they possess a sufficiently complex neural structure to enable felt pain plus an evolutionary need for such consciously felt states. They possess complex and specialized organizations of tissue call sense organs, they possess a specialized and complex structure for processing information and for centrally orchestrating appropriate behaviours in accordance with mental representations, integrations and reorganizations of that information. The proper attribution of felt pain in these animals is well justified, but it is not for plants by any stretch of the imagination.