I once did research for a professor who was writing a book on animal ethics, so I recently had occasion to read every academic article I could get my hands on that justifies consuming animal products. It turned out not to be a very taxing project, as I could find just nine of them. Most of the articles use one of the three strategies I detail below. There are some other interesting non-academic articles worth mentioning, but they too usually fit into one of these three categories. (And there are some other arguments people have made, which I'll cover some other time.)
The challenge for anyone who wants to justify eating animals is to say why all humans, but only humans, can't be eaten while everything else can. Most philosophers think that one's species shouldn't matter: it's as arbitrary as one's sex, height, or ethnicity. So we need a theory that is broad enough without being too broad. Here are some attempts, with my thoughts on the success of each. My goal here, however, is not to give each one thorough treatment: I'm more interested in the taxonomy of justifying meat eating.
Animals Are Not Us
These arguments proceed in the following way. Humans have some unique capacity (rationality, the capacity to plan for the future, empathy, etc.) that no other species has. Therefore, humans are justified in excluding other species from moral consideration. One common form is called contractualism (because the resulting moral system is based on a hypothetical contract). It holds that morality is something rational agents agree to, so only rational agents fall within the scope of consideration. The reason rational agents agree to the contract is that it guarantees the fairest outcome for everyone, and it preserves peace and social stability. Non-human animals aren't rational agents, so they don't count.
The most common criticism of contractualism is that some humans aren't rational, including the very young, the very old, and those born with cognitive impairments. But contractualists don't want the theory to allow the eating of humans, so they need some way of avoiding this worry. The problem is that if we make the circle big enough to include such outliers, lots of non-human animals will need to be included also.
Peter Carruthers, a philosopher at the University of Maryland, argues that we can avoid the conclusion that eating non-rational humans is morally permissible by appealing to the goals of the hypothetical contract. The reason rational agents enter into the contract is to preserve peace and social stability. Eating non-human animals is never going to cause direct social instability, but eating other humans will. No matter how sound the argument is, society will never allow for eating other humans, and the result if we allow it will be social instability and violence. Therefore, contractualists can defend eating non-human animals without the risk of justifying eating humans. Of course, the success of Carruthers's argument rests on our acceptance of contractualism which, despite having positive features, is also controversial in various ways. But I won't go into the arguments for and against contractualism here.
Theories that Seem to Prohibit Eating Animals Actually Don't
This category justifies eating animal products by arguing that the theories that were originally thought to prohibit eating animals actually allow animal consumption. The two most famous philosophers who argue against eating animals are Peter Singer and Tom Regan. Singer is a utilitarian: he argues that the wrongness of eating meat comes from the harm done to animals through factory farming and other agricultural practices. Our moral duty is to maximize the good. For Singer, there is nothing intrinsically wrong with killing a non-human animal—wrong only occurs when the animal feels pain. To avoid causing pain to animals we should avoid supporting any industry that does so. Therefore, we ought to be vegan.
Regan, in contrast, is a deontologist: he believes that the wrongness of eating meat comes not only from causing pain to the animal (although that is part of it), but from using it merely as a means to our ends. Our agricultural practices fail to respect the rights of animals which they possess in virtue of their capacities—capacities which humans and animals both share. In situations where our actions will cause harm to an animal and there's no other option, Regan says that we ought to choose the option that will cause the least harm overall. (Utilitarians and deontologists agree on this point.) Forswearing animal products produces the least harm, therefore that's what we ought to do.
However, there are challenges to the claim that veganism produces the least harm. One such challenge comes from Steven Davis, a professor of animal science at Oregon State University. Davis argues that monocrop agriculture (which most vegans rely on) kills more animals than a model that combines intensive agriculture with the pasture/forage model in which ruminants (such as cows) are allowed to graze. How can this be? Apparently, millions of animals are killed every year when fields are plowed and harvested, often in very high amounts. (For example, mowing alfalfa causes the gray-tailed vole population to decrease by 50 percent.) Of course, we would still need to produce some crops to eat, but if Davis's numbers are correct decreasing the amount of intensive agriculture is morally required. The best balance remains to be determined.
It's worth noting that Davis's solution still requires abandoning the status quo. In fact, we're acting in the worst possible way right now by intensively growing crops such as corn and soy to feed to factory farmed animals such as chickens which we eventually kill. And because chickens and pigs aren't ruminants, society will have to give up eating them altogether. Davis's model requires getting rid of factory farms, because they rely on intensive agriculture. So there will be less meat available, and it will certainly cost more, but it's the system that produces the least harm.
Davis is assessing Regan's least harm principle, but recall Singer, who thinks that we ought to maximize the good (which includes minimizing suffering). As I said, Singer thinks the wrongness of eating meat comes primarily from the conditions the animals are kept in, because factory farms cause the most suffering. But were a system to be developed that allowed the animals to live pain-free lives until they are painlessly killed, utilitarianism has no problem with such a system.
Although Davis's system might produce the least amount of harm to animals, his calculations don't consider other morally relevant factors, such as environmental damage. It's easier to minimize the environmental damage of growing crops than it is when growing animals. The latter require more water and produce high levels of waste in the form of feces and methane. So it might not turn out that lots of ruminants are worth it after all. We would to calculate the number of animals deaths caused by environmental damage caused by ruminant agriculture.
But as it turns out, we don't need to wait for such a study. Gaverick Matheny of Johns Hopkins has pointed out some significant errors in Davis's analysis. In particular, Davis errs by assuming that the same amount of land would be needed in the crop only system as it would be in the combined one in order to feed the same number of people. Here's Matheny:
In fact, crop and ruminant systems produce different amounts of food per hectare – the two systems would feed different numbers of people. To properly compare the harm caused by the two systems, we ought to calculate how many animals are killed in feeding equal populations – or the number of animals killed per consumer.
Davis suggests the number of wild animals killed per hectare in crop production (15) is twice that killed in ruminant-pasture (7.5). If this is true, then as long as crop production uses less than half as many hectares as ruminant-pasture to deliver the same amount of food, a vegetarian will kill fewer animals than an omnivore. In fact, crop production uses less than half as many hectares as grass-fed dairy and one-tenth as many hectares as grass-fed beef to deliver the same amount of protein. In one year, 1,000 kilograms of protein can be produced on as few as 1.0 hectares planted with soy and corn, 2.6 hectares used as pasture for grass-fed dairy cows, or 10 hectares used as pasture for grass-fed beef cattle (Vandehaar, 1998; UNFAO, 1996). As such, to obtain the 20 kilograms of protein per year recommended for adults, a vegan-vegetarian would kill 0.3 wild animals annually, a lacto-vegetarian would kill 0.39 wild animals, while a Davis- style omnivore would kill 1.5 wild animals. Thus, correcting Davis’s math, we see that a vegan-vegetarian population would kill the fewest number of wild animals, followed closely by a lacto-vegetarian population.
An altogether different strategy comes from Jonathan Harrison, who has been retired since 1988 but is still philosophically active. In his paper “The Vagaries of Vegetarianism,” Harrison points out that each person's conversion to vegetarianism will make almost no difference to the number of suffering animals. (Before proceeding, Harrison begins by calling it a “fun paper,” but he still makes some interesting points even if he intends his remarks to be taken less seriously.) Agreeing with Singer that no harm comes to an animal when it is killed painlessly, the vegetarian movement needs to focus on decreasing levels of suffering, not decreasing meat consumption. Buying meat but not eating it makes no difference to the amount of suffering the animal experienced.
In contrast, eating meat but not buying it does decrease animal suffering. It's worth quoting at length:
Hence, if I had a charismatic personality, and wished to put an end to meat producing, I would start a campaign to persuade people never to buy meat. They would be encouraged to eat it, provided they begged, borrowed or stole it, but did not pay for it. In fact, the more meat they stole the better. A kind of boycott in which everyone stole and ate meat, but no-one paid for it, would bring the meat-producing industry to an end much more quickly and effectively than only a few eccentric people's just not eating meat. Even only a fairly small proportion of people eating meat they did not pay for might make meat producing uneconomic, though almost everybody would have to stop eating meat to produce the same effect. … (I am not optimistic about this helpful suggestion ever being acted on.)
Actually, nothing important turns on the eating of the meat after it's stolen, perhaps besides that people will feel more motivated to steal it in the first place. Harrison's is certainly the most creative solution I've come across.
It Depends On What You Mean by Meat
The final approach also disputes the conclusions of moral theories that attempt to justify full-time vegetarianism, but it does so in a different way. If the wrongness of eating meat comes from the harm done to animals, we can eat meat so long as the animal can't feel pain. In an article in Slate, Christopher Cox describes how he's vegan with one exception: he eats oysters. The two arguments that justify veganism—eating animals causing needly suffering and destroys the planet—don't apply in the case of oysters.
Oysters most likely feel no pain (they have no central nervous system), so they can't be harmed. And oyster farming causes no secondary killing because, unlike clams and mussels, oysters aren't farmed by dredging the ocean floor. The reason vegans draw the line at eating animals is because animals, and not plants, feel pain. Oysters are the exception. So the fact that they are animals should make no difference to our moral sensibilities. Cox puts it as follows:
When I became a vegan, I didn't draw an X through everything marked "Animalia" on the tree of life. And when I pick out my dinner, I don't ask myself: What do I have to do to remain a vegan? I ask myself: What is the right choice in this situation? Eating ethically is not a purity pissing contest, and the more vegans or vegetarians pretend that it is, the more their diets start to resemble mere fashion—and thus risk being dismissed as such. Emerson wrote, "A foolish consistency is the hobgoblin of little minds." A foolish consistency: If you resolve to give up foods that begin with the letter B, and if you stick to that for the rest of your life, you'll be mighty consistent. You'll even benefit the world by cutting out beef. But there's no good reason to avoid broccoli—unless, like George H.W. Bush, you don't like the taste. There is, on the other hand, great reason to be an inconsistent vegan and make an exception for oysters—for it is surely foolish to deprive yourself of an icy plate of white-shelled Watch Hills.
A different option, made possible by advances in technology, is the production of synthetic meat. This meat is real—that is, it's actual animal muscle tissue—but instead of taking it from an animal, it's grown in a lab. There are different ways of producing such meat, but the details aren't important here. What's important is that we are on the cusp of being able to eat any type of meat we desire without worrying that an animal was harmed in the process.
Patrick Hopkins and Austin Dacey lay out the reasons for desiring such a solution. Currently, many meat eaters think they would be acting morally better if they gave up meat. Yet despite their attempts, they are unable to do so. Synthetic meat offers a solution: meat eaters can go on eating meat, and animals don't have to suffer.
Hopkins and Dacey discuss possible objections to the practice, but they find all of them unconvincing. There are some moral theories that would condemn eating synthetic meat, but most—including utilitarianism and many forms of deontology—have no problem with it.