“Every day, thousands of innocent plants are killed by vegetarians. Help end the violence. Eat meat.”

These words, written last month by an Edinburgh butcher on a blackboard outside his shop and shared on a vegan Facebook group, led to a heated online discussion. Some condemned the butcher for seeking to blur an important line between beings capable of suffering and those that are not. Others took it as a joke, as the butcher said he had intended it. But jokes can make serious points.

“How do you know that plants can’t feel pain?” I was often asked when I stopped eating meat. In 1975, in the first edition of “Animal Liberation,” I offered two distinct responses. First, I argued, we have three strong reasons for believing that many nonhuman animals, especially vertebrates, can feel pain: they have nervous systems similar to our own; when subjected to stimuli that cause pain to us, they react in ways similar to how we react when in pain; and a capacity to feel pain confers an obvious evolutionary advantage on beings able to move away from the source of the pain. None of these reasons applies to plants, I claimed, so the belief that they can feel pain is unjustified.

My second response was that if plants could feel pain, even if they were as sensitive to it as animals, it would still be better to eat plants. The inefficiency of meat production means that by eating it we would be responsible not only for the suffering of the animals bred and raised for that purpose but also for that of the vastly larger number of plants they eat.

That second response clearly still stands. Estimates of the ratio of the food value of the plants we feed to animals to the food value of the edible meat produced range from 3:1 for chickens to 25:1 for beef cattle. I don’t know if anyone has ever tried to calculate how many plants a cow eats before being sent to market, but it must be a very large number.

Increasing interest in plant sentience, however, has cast some doubt on my first response. Peter Wohlleben’s 2015 worldwide bestseller, “The Hidden Life of Trees,” sparked popular attention to the issue. Wohlleben, a German forester, writes that trees can love, fear, make plans, worry about future events and scream when they are thirsty -- claims that have been repudiated by many scientists, some of whom signed a petition with the heading, “Even in the forest, it’s facts we want instead of fairy tales.” When questioned, Wohlleben himself often backs away from his attributions of mental states to plants.

That plants are sentient, in the literal meaning of the word -- able to sense something -- is obvious from the fact that they grow toward sunlight. Some are also sensitive in other ways. As a child, I enjoyed touching the leaves of a Mimosa pudica, or “touch-me-not” bush, that my father had planted in our garden, to see the leaves close in response. And the carnivorous Venus flytrap has sensitive hairs that trigger the trap when an insect touches them.

But is there something that it is like to be a plant, in the sense that there is something that it is like to be a chicken, or a fish, or (possibly) a bee? Or is being a plant like being a rock -- in other words, there is no subject of experience?

In “Animal Liberation,” I argued that plants are like rocks, and not like chickens or fish. (I was agnostic about bees, though I have not been indifferent to the question). Isn’t it possible that my argument -- that we frequently underestimate the awareness, needs and cognitive abilities of animals, especially those we want to use for our own ends -- applies to plants, too?

Consider the three reasons I gave for believing that animals can feel pain, which I claimed do not apply to plants. Both the fact that plants do not show pain behavior, and the apparent absence of an evolutionary advantage to consciousness for stationary organisms, could be met by the claim that they do respond to stresses, but on a much longer timescale than animals. They may not have a central nervous system, nor the neurons that form the physical basis of consciousness in animals, but they have substances like dopamine and serotonin, which function as neurotransmitters in animals.

There is still much that we have to learn about both plants and consciousness. At this stage of that learning process, it would be foolish to exclude the possibility that plants have some physical basis for consciousness that we do not know about.

This does not vindicate the Scottish butcher’s justification for eating meat. Not only does the second of the responses I made in “Animal Liberation” still stand; we now know that eating plant-based foods will significantly reduce our contribution to climate change. But it is a reason for thinking about plants a little differently, keeping in mind the possibility that more may be going on than we are aware of and acting accordingly by minimizing the harm we do to them, when the costs of changing our behavior are not significant. On a larger scale, of course, we also know that forests and other forms of vegetation are essential for preserving biodiversity, not only for ourselves but for other animals as well.


Peter Singer
Peter Singer, a professor of Bioethics at Princeton University, is the author of “Animal Liberation.” -- Ed.

(Project Syndicate)