What About Plants?

Original text Luciano Carlos Cunha · Volume IV, p. 33 of the PDF edition · English translation

A very common argument put forward in defense of consuming animals claims that whatever could make it wrong to consume animals implies that it would also be wrong to consume plants, and so it makes no difference whether we consume one or the other. Let us call this argument the plant objection. It comes in two variants, discussed separately below.

4.1. The "plants are alive too" variant

The first variant of the plant objection says that it makes no difference whether we consume animals or plants, since in both cases we will be causing deaths.

This objection is often answered by pointing out that if one's goal is to kill the smallest number of living organisms, then one should give up consuming animals, since a large share of the plants grown in the world is used to fatten the animals that will be killed¹⁰³. There are two problems with this reply. The first is that the defender of consuming animals could respond that if we ate only aquatic animals caught in the wild at sea, we would kill even fewer organisms, since no crops are used to fatten these animals. The second problem is that the reply does not address the assumption, present in the objection, that if death is a harm for animals, it is also a harm for plants. As we will see below, this assumption is mistaken.

The reason death is a harm is not that it stops biological life from continuing, but that it prevents the individual from going on to enjoy the positive things they would experience if they stayed alive. This shows that death harms only sentient beings, since only in their case is there someone inhabiting that body (an individual, a consciousness) whom death would deprive of enjoying something positive. Living beings that lack consciousness are living bodies. There is no one living those lives. So there is no one there to be harmed by death.

It might be objected that each and every life, even with no possibility whatsoever of sentience, is always valuable. However, the following thought experiment suggests that for a life to be valuable, there must be the possibility of experiences occurring in that life.

Imagine that someone assures us that they can give us an afterlife if we pay a good sum every month until the end of our lives, and guarantees that the future life will be valuable for us. Suppose, however, that after paying our whole lives, we discover that we will be reborn as living organisms, but with no possibility whatsoever of sentience. If in that case we have been cheated, it is because a merely biological life, with no possibility of experiences at all, is not valuable¹⁰⁴.

Thus, the reasons against killing animals do not imply that there are reasons against killing plants; quite the opposite. Therefore, the first variant of the plant objection fails.

4.2. The "plants are sentient too" variant

The second variant of the plant objection maintains that plants are sentient, and that we are therefore justified in consuming animals since, whatever we choose to consume, we will be killing sentient beings (who would therefore be harmed by death). This variant has two problems, which we will discuss below.

The lesser problem

The lesser problem is that, most of the time, the claim that plants are sentient rests on the mistaken idea that behavioral evidence is a sufficient indicator of sentience. For example, in support of the view that plants are sentient, it is often mentioned that they display a range of behaviors. For instance, some plants close up when touched, such as the sensitive plant and carnivorous plants. In addition, plants perform countless functions related to absorbing nutrients from the soil, carrying out photosynthesis, and so on.

The problem with this is that the presence of behavior does not, by itself, show that there is sentience. There can be quite complex behaviors that are nonetheless merely physicochemical stimulus-response reactions, with no one internally experiencing what is happening. Even in the case of animals, behavior is not, by itself, sufficient evidence of sentience. This is because of reflex arcs, explained below.

When information from an external stimulus reaches a nervous system, it travels along two pathways. One of them, the shorter, goes straight to the muscles, producing the reaction that is a reflex (hence the name reflex arc)¹⁰⁵. The other, longer pathway goes to the brain, where the information is processed and the conditions for experience to take place are created. For example, when we touch a hot plate with our hand, we pull the hand away instantly, but only a few microseconds later do we feel anything. A muscle can keep displaying the reflex even if the sensory part has been cut off (for example, by means of local anesthesia)¹⁰⁶. Some kinds of nervous systems operate solely with the reflex-arc part. This is the case of animals that have nervous systems but no centralizing organ (no brain, no ganglia), such as cnidarians (jellyfish, for example) and echinoderms (such as starfish and sea cucumbers). These animals may even display quite complex behaviors, but solely through reflexes, without anyone there experiencing anything. In the case of plants, there is not even a nervous system. So it is very likely that their behaviors, of whatever kind, are merely the result of physicochemical reactions.

Thus, the main criterion for assessing whether a being is sentient is the presence of a centralized nervous system (a brain or ganglia, for example), since it is the processing of information that takes place in the centralizing organ that provides the conditions for consciousness to arise¹⁰⁷. If an organism has such a structure, the way it behaves can be a good additional indicator that it is sentient, but only if the best explanation of its overall behavior is to assume that it is avoiding negative experiences and seeking positive ones. For this reason, the behavioral evidence useful for determining whether or not a being is sentient is not evidence about behavior in specific situations, but the way it behaves in general. In the case

of plants, besides the fact that they do not even have a nervous system (let alone a centralizing organ), it seems that the best way to explain their behaviors is not by assuming that they are sentient. We will see more about this below.

The traits most likely to prevail over generations are those that give an advantage in terms of passing on genetic information. For example, if a given trait helps an animal survive, it increases the likelihood that the animal will also reproduce and thus pass that trait on. If sentience gave its bearers no advantage at all, it would most likely not have been passed on, since it requires a considerable expenditure of energy. For example, in humans, around 20% of the organism's energy is spent maintaining the structures associated with the brain¹⁰⁸. Thus, sentience probably prevailed because it gave its bearers some survival advantage. Otherwise its bearers would probably have perished without leaving descendants.

The evident advantage is that sentience is associated with the motivation for behavior: a sentient being tends to avoid negative experiences and seek positive ones. Since animals can move about in their environment, this capacity increases their chances of survival and, consequently, of passing on their genetic information. This is not the case for plants, however, since they cannot move. In their case, sentience would involve a considerable expenditure of energy with no advantage at all. Therefore, it is very unlikely that such a trait would have prevailed in plants or fungi, for example¹⁰⁹.

Thus, to maintain that plants are sentient, it would have to be shown that: (1) having a nervous system with a centralizing organ, or a structure performing a similar function, is not necessary for sentience; (2) behavioral evidence is sufficient for sentience; and (3) traits that involve a considerable expenditure of energy and give their bearers no apparent evolutionary advantage can nonetheless prevail over generations. This is, at the very least, very hard to demonstrate.

This does not mean that it is impossible to meet these conditions. It does mean, however, that the burden of proof lies with those who claim that plants are sentient. For example, regarding the first

condition, it would have to be explained by what other mechanism sentience could arise that would make the nervous system unnecessary. It would also have to be explained, then, what the role of the nervous system would be, and so on.

One of the factors that sometimes leads people to believe that plants are sentient is that there are many studies of plants and fungi that use terms such as "learning," "memory," "communication," "experience," and so on. In most of these studies, these terms are used to refer solely to behavior, without claiming that sentience is present. In ordinary language, however, these terms are associated with sentience; that is, they are used in a way that indicates there is someone there learning, memorizing, communicating, experiencing, and so on. So it is possible for a given study of plants or fungi to use these terms in a sense that does not imply sentience, and for someone who reads the study not very carefully to think that it is a study demonstrating sentience in plants or fungi.

The greater problem

The greater problem with the argument is this: even if it had been shown that plants are sentient, the practical implication that would follow is the opposite of what the argument's proponents conclude. Recall that the claim that plants are sentient is usually brought up in an attempt to justify consuming animals. However, if plants were sentient (or were morally considerable for some other reason), what would follow is not that we are justified in consuming both animals and plants, but rather that we ought to be ethical botanical fruitarians.

This stance is characterized by consuming only what detaches from a plant. Besides everything usually called fruit (such as bananas, oranges, apples, strawberries, and so on), this includes nuts, seeds, grains, legumes, and vegetables such as tomatoes, pumpkins, zucchini, bell peppers, eggplant, and so on.

Note that the implication that we should practice fruitarianism would hold whether one maintains that plants are sentient or that plants should be morally considered because they are alive. That is why, whichever variant of the plant objection we are talking about, it fails to justify consuming animals.

Usually, when people realize that the plant objection implies fruitarianism, they immediately stop defending it. This reveals that they had no real concern for plants: it was just an excuse to keep consuming animals. Regardless of this, however, the important point to note from the above analysis is this: whether it is raised hypocritically (as is usually the case) or out of a serious concern for plants (as is rarely the case), the plant objection fails to justify consuming animals.