James:
I'm happy to. I don't see much value in merely throwing around value judgments and ethics is always an interesting topic.
Having said that, it is important to remember that in the real world it is hardly ever, if ever, a question of "the cow dies or I die". If we restrict our consideration to affluent, western societies, then every person ordinarily has a choice of what they consume to sustain their own life. Choosing to consume another sentient being instead of, say, a plate of vegetables then requires further justification than "I need to eat to or I'll die".
Yes, I intended to get to this eventually because I think it's central to the debate. Until very recent times it has been primarily a question of starvation and pure survival.
I know this is rather obvious, but it bears stating that animals have the ability to convert materials such as grass that humans cannot digest into meat, fat, milk, and eggs which we can subsist on. Animals may also be kept and slaughtered in deep winter when little is available in the way of fruits and vegetables. This allowed us to inhabit areas and climates that would have otherwise been impossible to live in. Even today there are many areas of the world where humanity finds it difficult, if not impossible, to sustain its population without the addition of meat. That human morality evolved with a rather low intrinsic value assigned to animals is not arbitrary, it's based upon survival.
Subsequently, I think it is fair to note that we are talking about changing what has historically been a valid moral point of view. The question becomes, if it can be reasonably supported that a human does not have to eat meat in order to survive is it immoral?
At some point we're going to have to address ethics in general, where values originate and the legitimacy of their arguments (is "natural" behavior ethical, is killing immoral, etc.) but I'm going to skip this more difficult problem for the moment to address a salient point.
One of the main assumptions of what I'll call the vegetarian side of the argument is that between the two only meat eaters are responsible for killing animals for food. But while there is a direct and obvious link between eating animals and killing them there is a less obvious but none the less causal link between vegetarianism and the death of animals. Vast swaths of natural ecosystems have been utterly destroyed in order to provide grains, fruits, and vegetables for humans. Chemical and biological warfare is committed against "pest" species in order to ensure the human food supply. Runoff from horticultural activity as well as things like the depletion of the water table, pollution from machines, and other unintended consequences of growing crops.
All of these things cause, directly or indirectly, the death of animals in the preferential interest of humans. Even a vegetarian living in the wild, eating nuts, roots, and berries is competing with other animals for food. There is really no escaping the fact that at some level, deliberately or not, we will cause the death of other animals.
Often the response to this is, "modern agriculture is also immoral" or "we should reduce the human population". To which my reply is, "okay, but you go first" which usually reveals the hypocrisy of the position of asserting that others should make sacrifices you are not willing to make yourself. It's easy to assert a moral position when you do not have to concern yourself with consequences other people would have to face, but it's no less hypocritical than a meat eater proclaiming they do not kill any animals themselves.
When you start comparing what you call the intrinsic value of an animal to its value as food for a human being, then you need to make sure you're being ethically consistent. How do you rank the intrinsic value of a human being compared to his or her intrinsic value as food for another human being or for an animal? Would it be permissible to breed human beings solely to be eaten by other human beings? If not, why not? And if not, what is the morally significant difference between the cow and the human that makes such behavior acceptable in the case of the cow?
Here we start getting deeper into ethics. I'll try to avoid needless complications and address the questions as directly as I can.
As to why a human is more valuable than a cow, I find there are several arguments. One is that generally speaking, a human has a vastly greater potential to contribute to the overall good of every living thing. A human may fight for the benign treatment of cows, invent an antibiotic, rescue a puppy, or stop a forest fire. No cow has or ever will be able to do such a thing. The most universally beneficent act a cow has ever done on its own is protect another cow from a predator. Of course this is a general evaluation, specific individuals may differ greatly and I'm not opposed to the notion that a specific human life may be less valuable than a cow's depending on the circumstance. But as a principle of general value, I believe it withstands scrutiny.
A second argument I draw from the Social Contract theory of ethics, which I personally give a lot of weight to. From this perspective morality is (in the present reality) almost exclusively a human concern with perhaps some consideration due towards certain companion animals. A cow has no direct placement in such a scenario, it's value cannot be assessed aside from human interests.
As to why it is unacceptable for a human to kill another human for food, the same principles apply. From a general evaluative standpoint they are equal and engaged in a social contract with one another. Part of that contract is that humans are not supposed to kill one another.
More broadly, we have a tendency to provide the same status to every human whether they are specifically able to live up to the all the standards of a social contract. Here is where I find your question of personhood comes into consideration. Since animals are obviously not literally people I find the onus of the argument lies upon you. Towards that end I will state that I find significant differences between various species, but to merit the case in one instance does not provide sufficient argument across the board. In other words, you'll have to show me why I should consider a cow to be a person even though I might be willing to grant that status to a mountain gorilla (I don't eat mountain gorillas). You may also be able to provide reason under the Social Contract theory as to why I shouldn't eat dogs, but that does not apply to fish.
The moral question then becomes: does your personal pleasure in consuming an animal (which is not in any way necessary other than to satify a desire you have) outweigh the animal's interest in continuing its existence? If you think it does, you really need to justify that.
I don't find it merely an issue of personal pleasure, I was just establishing a boundary at that end.
While it is possible to sustain a healthy vegan diet in a modern industrialized country it needs to be rather intensively planned, it is costly, it is dependent upon supplements, and is generally unavailable locally. Along with any current environmental issues due to agricultural development and transport much more farmland would be needed to offset the difference.
In my opinion the impact to human life and the overall environment offsets most of the negative moral implications. Need is still a primary factor. Certainly consumption in some modern countries goes quite beyond mere dietary considerations but in much of the world nutrition is still a very serious problem and removing meat from people's diets would make things even worse.
As to if these issues were resolved and eating meat was only an issue of pleasure, I'm not quite sure. I believe I would still find it acceptable to eat some animals, though I think I would want to reduce my consumption even further than I already have. In this instance, ongoing advances in making vegetable products more satisfying would probably go a long way towards eliminating meat consumption.
Going back a step, it's important to realise that some people don't believe that non-human animals actually have any interests at all, and so no moral considerations apply in respect of them. The question of eating them is then a non-issue. Killing an animal is no worse in a moral sense than breaking a stone in half. Neither has any moral weight.
I find this problematic too. Everything has some intrinsic value, even if it is outweighed by mere whim. I have not the slightest ripple of moral consideration when removing a smooth pebble from the beach but I am still dumbstruck when I consider how it came to be there. Certainly every life has some measure of value.
~Raithere