“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.