Is it right to kill animals for food? And if it's wrong, how wrong is it? Could and should Western society ever change its views?
Four philosophers share their views with BBC Radio 4's Analysis programme.
Peter Singer: Our future selves will consider meat eating to be barbaric
Peter Singer is professor of bioethics at Princeton University and the author of Animal Liberation.
"You could say that if you kill a cow you're depriving it of the rest of its existence, which could also have been a happy, good existence, so why deprive it of that just because you want to eat some meat when you've got other healthy, nutritious, delicious things that you could also eat?
"The counter-argument is this cow would not have existed if we had not already planned in advance that at some point we would kill it and we would sell the meat, because obviously it costs something to rear a cow, and we can only meet that cost if we are going to kill it.
"So in a sense the cow could thank us for her existence - at least she has some existence rather than none.
"If a cow is killed that will make it possible for another cow to come into existence who will have a good life, and if the first cow were not killed it would not be possible for the other cow to come into existence.
"So yes, this cow standing in front of us will lose the rest of her life, but that loss is replaced by bringing the other cow into existence and the other cow will also have that happy life.
"In theory - other things being equal - I do buy that argument. I say in theory because I think it's very hard to produce circumstances where that actually occurs and there aren't other undesirable side effects. Given the animals in our food supply are mostly cattle and sheep, and they are major producers of greenhouse gases, I think on balance, it would be better if they didn't exist.
"I think we'll come to view [eating meat] in the way we now look back on the Roman games; having crowds of enthusiastic people cheering on the lions as they slaughtered the Christians or gladiators fighting each other to the death.
"The last time I intentionally ate meat was 1971. I grew up eating a lot of meat in Australia and I liked it, but I really haven't missed it for a long time."
Elizabeth Harman: A moral mistake but not morally wrong
Elizabeth Harman is associate professor of philosophy in human values at Princeton.
"The kind of moral picture that I would urge is one in which we think about whether we can justify our treatment of individuals. If you're going to do something terrible to a particular morally significant individual, how can that be justified?
"Animals have moral status, and animal suffering matters because it's a harm to something that counts morally. Killing an animal harms the animal. We're actively doing something that deprives it of future life.
"One way of thinking about how to justify an action is what could you say to the one that you're harming? That works very well with people. It works less well with cows who can't understand justifications.
"But we can imagine someone who is a representative of the cow and what could you say to the cow's representative to justify your treatment of it? If you kill one cow and then you create another cow, that doesn't justify killing the one cow at all, in my view.
"I think that meat production is morally wrong, and I think that eating meat is a moral mistake but not morally wrong.
"If you buy or eat meat, you're doing something that plays some kind of causal role in meat production, but it's a very removed causal role, so it's not plausible that any particular animal suffering depends on whether you make a particular purchase. So in that way you don't have any particular bit of animal suffering or death on your head for that instance of meat eating.
"What you're doing is participating [in] the continuation of meat production, and you're also failing to participate in the vegetarian movement, which I think is a really good moral thing that's happening.
"I do still eat meat and I'm really torn about how I feel about that."
Jeff MacMahan: Cows have a moral interest in continuing to live
Jeff MacMahan is White's Professor of Moral Philosophy at the University of Oxford.
"If you didn't kill the cow, it could go on living and have a life that would be good for it. That's part of why it would be wrong for me to kill you. It would be depriving you of the good experiences that you would have if I didn't. And an animal has an interest in living to have its next meal as well.
"You don't have to think about humans in exactly the same way that you think about cows. But you've got to explain why you think it's permissible to do to an animal what you think it would be impermissible to do to a human being. In the case of people their suffering matters, but their happiness also matters. The same should be true in the case of animals.
"Do I think it's permissible to kill a cow and eat it if it has had a life that's been good and it's killed painlessly?
"If people couldn't get adequate nutrition otherwise, then I think yes. In a society like contemporary America or Great Britain my inclination is to say no. It's not at all clear that the interest that people have in killing and eating the cow outweighs the interests that the cow has in continuing to live.
"That's because the flesh of the cow is going to provide a certain amount of additional pleasure for the people who eat it. When we do the cost benefit analysis here, we shouldn't weigh all the pleasure that people get from eating a meal with the cow's meat in it; we should weigh just the difference in pleasure that they would get from eating the cow and eating some meal that didn't have meat in it.
"My own view is that in most cases that's not very much. I don't feel as a vegetarian that I'm in any way deprived."
Gary Comstock: Cows can learn - and know they're doing so
Gary Comstock is professor of philosophy at North Carolina State University.
"In killing an animal we deprive it of its ability to have a future and to satisfy its desires, so I'm very interested in whether cattle look forward. I think they do.
"An interesting experiment was done by Donald Broom and colleagues at Cambridge a few years ago with heifers - one-year-old cows - which seems to show not only that they can learn, but that they take satisfaction in knowing that they are learning.
"In the first control group, heifers learned to hit a button that let them into a long chute, at the end of which was a reward. They knew which button to push, but had no control over when the gate would open. They were interested to get to the end of the chute, but otherwise their behaviour was unremarkable.
"A second group of heifers had control over when the gate would open. As they learned which button to push to open the gate, they got better at pushing the right button, and the gate opened faster so they got the reward more quickly.
"When the second group of heifers saw their own improvements in performance, they jumped and kicked and galloped down to get the reward, behaviours that suggest strongly that they not only anticipated the pleasure of the coming reward, but were also taking pleasure in their own role in making it happen. It seems they were aware of - perhaps even proud of - their accomplishment.
"There is a danger of anthropomorphising the animals, of over-interpreting their behaviour, [but] these are controlled experiments, these are not my intuitions about what I think is going on in cattle's minds.
"There is also anatomical evidence. If you look at the brains and neural pathways in cattle, and compare them to humans, there are massive similarities. The amygdala, the cerebellum, the thalamus which are all involved in processing pain in us, are all found in cattle.
"People used to justify eating meat for biological reasons: we are omnivores, our incisors are designed to eat meat, this is a natural thing for us to do. The problem is there are many natural things that are not right for us to do, and the biological features are irrelevant to the question of how we ought to live our lives.
"Evidence seems to be building that the shoe's on the other foot now; that those who want to kill animals and eat them ought to justify their view. It shouldn't be the other way round."
Analysis is on BBC Radio 4 on Monday 26 October at 20:30 GMT. You can listen online or download the programme podcast. At 21:00 GMT on Monday 26 October, Jeff MacMahan and Gary Comstock will take part in a live Q&A on the Radio 4 Facebook page.
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