Vegetarianism

Vegetarianism is the purposeful exclusion of meat from a person's diet. I have never been a vegetarian, and, having grown up in a meat-eating family and having developed a love of meat, I probably never will. However, despite not being a vegetarian, I have to admit that vegetarianism is the morally superior option, and I think I can effectively argue my case even among some of the more exotic ethical systems.

Argument

P1: It's immoral to purposely kill an intelligent being that doesn't want to be killed (without a sufficient reason).
P2: The more intelligent a being is, the more it is injured by being killed, and the more immoral the killing.
P3: The reasons given for purposely killing a being for food are rarely sufficient.
C1: Therefore, it is usually immoral to kill a being for food.

I'll first elaborate on each premise, then discuss common objections below.

P1: It's immoral to purposely kill a being that doesn't want to be killed (without a sufficient reason).

For the ethical systems I've encountered, this is an accurate statement. It is the basis for murder laws, animal cruelty laws, and so forth. Everyone I've ever met would agree that killing a human just for fun is immoral, and the vast majority would agree that killing an animal for fun is also immoral. Obviously, there is some wiggle room with the "sufficient reason" clause, for example, if the being is a threat to another life.

P2: The more intelligent a being is, the more it is injured by being killed, and the more immoral the killing.

Immorality of Killing an Intelligent Being.

This is probably the most important premise of my argument. People generally tend to accept this at face value, for example, they feel far worse about the killing of a dog than the killing of a mosquito, however, I think it's important to understand why this is true, as it gets to the heart of the question, "why is murder wrong?" In general, the more intelligent a being is, the more they have to lose from being killed. An insect has little in the way of a central nervous system, while a human has desires, goals, dreams, etc., all of which are subverted when they're killed. Likewise, when an ant is killed, its fellow ants don't appear to be bothered by its death, but, when a human is killed, all their loved ones suffer as well. These are both extreme cases, but we can see how a chimpanzee has less to lose than a human, a pig has less to lose than an chimpanzee, and so forth. We may disagree on the actual level of "loss" and "injury" for each animal, but the fact that it exists is all that matters to prove this premise.

P3: The reasons given for purposely killing a being for food are rarely sufficient.

I'll address the reasons in the "Objections" section below.

C1: Therefore, it is usually immoral to kill a being for food.

This conclusion follows if the three premises are true. Note that I'm not saying that every conceivable act of killing an animal for food is immoral, just that the common arguments listed are insufficient.

Objections

Here I deal with objections to my argument as well as common objections to vegetarianism in general.

We can kill animals humanely.

While I agree that painless killing is better than painful killing, it doesn't change the fact that the being didn't want to be killed in the first place. We wouldn't accept that a person who is murdered in their sleep wasn't injured, because we know that they wanted to remain alive. Such a desire to live is also found in non-human animals, and while it may be difficult to demonstrate with an insect, it's quite clear with a chimpanzee.

Animals aren't intelligent enough to care about death.

We must first answer the question, what does it mean to be intelligent enough to care about death? We know that some animals make and reuse tools, that they can solve multi-variable puzzles, that they can identify themselves in a mirror. Zoologists even have shown, pretty convincingly, that the more intelligent an animal is, the more signs of distress they show when an animal in their group is killed, and the more likely they are to risk their own life rescuing another in their group.

When asked the question, "Does the animal want to be killed?" we can safely answer "no," since all animals, even the less-intelligent ones, actively work to preserve their own life. And, while this seems purely instinctual in animals with less intelligence, it seems to be more of a conscious choice as the animal becomes more intelligent.

With our current understanding of neurology, while we may be able to drawn a line for when an animal becomes intelligent enough to care about death, it would be pretty arbitrary. This doesn't mean that we can't say definitely whether some animals can or can't understand death, but it does favor a gradient as described in premise 2.

Killing an animal for food is a sufficient reason.

I've never met anyone who would argue that it's okay to kill a human who doesn't want to be killed for food, but there are many who justify killing all lesser-intelligent animals. They're drawing a line on the chart above and saying, "it's only immoral to kill anything above this line." I understand wanting a binary simplification, but such a line is drawn arbitrarily and doesn't address premise 2: the more intelligent a being is, the more immoral it is to kill it. Regardless of where you draw the line, unless you can defeat premise 2, you're still choosing between two immoral options, and thus, immoral.

It's not immoral to kill an animal for food if you're starving.

First, ask yourself, "If I'm starving, is it immoral to kill another human who doesn't want to be killed and eat them?" Even if you might do it, you'll still probably admit that it's immoral, but perhaps not as immoral to kill and eat them if you weren't starving. In that same light, it's still immoral to kill a cow for food, even if you're starving, but perhaps not as immoral as if you have other food options. Also, in the developed world, people are rarely starving, and, even when they are, they rarely only have meat as a food option, and, if they do, it's probably due to a lack of foresight on their behalf.

I don't personally kill the animal, therefore I'm not immoral.

This is like saying, I hired a hitman to commit murder for me, therefore I'm innocent. By buying animals that were killed for food, you're culpable because you've created the market. Think of this on a smaller scale; imagine everyone on the planet is a vegetarian so nobody would kill an animal for food. But then, someone decides they want to eat meat, but they don't want to kill animals themselves, so they hire someone else to do it for them. Even though they aren't personally killing the animals, they are the sole reason the animals are killed.

Eating meat is healthier / Vegetarianism doesn't give proper nutrients / Etc.

We know that that there are millions of vegetarians world wide who have shown that humans can, fairly easily, thrive on a diet devoid of meat. However, even if a meat-rich diet were healthier, the argument doesn't address the morality of killing a being for food.

My god gave humans animals to kill for food.

Before such an argument can be properly addressed you should be able to properly define your god, and demonstrate how you know it created animals for human consumption. But, even if we assume this to be true, most religious people I know still behave as though purposely killing an intelligent being that doesn't want to be killed is immoral. They just turn a blind eye when it's for their own food.