I begin with a negative claim: I do not think it is always morally wrong to eat animals. It is natural and some groups of people (e.g. Eskimo living in the Alaskan wilderness) must eat animals to survive. It seems to me, however, that we have good reasons to refrain from eating some animals.
The special features that human beings possess are typically regarded as giving them special moral status. In philosophy we call this "personhood" and speak of those who have these properties as "persons." The relevant moral features for attributing personhood to a being are: Self-awareness, rational decision making, and the ability to think abstractly.
Some animals appear to have these faculties, perhaps not to the degree that we have and exercise them, but they have them to a degree. For instance, there is good evidence that Chimps, Dolphins, and some Whales have these capacities and are therefore "persons"
Of course most of us don't eat these particular animals - although they are often hunted and killed for human use - so it may not mean much. But we don't eat Dogs and Cats either (at least not in Western cultures) and pigs are as intelligent as dogs so why do we eat Pigs? We regard Dogs as too smart and too lovable to eat, if pigs are comparable on both these points, then we have no rational ground for eating pigs. If we lived in an environment where we had to eat meat to survive and pigs were the the only - or one of the few - animals we could kill and eat, this would be perfectly understandable. But we live in a technological society where we can live in full health and receive full nutritional value without eating Pigs. So what then is the case for eating them?
There is, in fact, a good case against eating mammals in general. The late Carl Sagan argued convincingly in The Dragons of Eden that "love seems to be an invention of the mammals" (67). Given the fact that mammals give live birth and their young need an extended period of care by their parents, love is an evolutionary necessity that begins with mammalian life.
We humans place a great value on love. We consider it the most wonderful thing we can experience, the meaning and value of our life. Many even declare that "God is love." Clearly the ability to love is something quite remarkable. Of course, lower mammals don't "love" in the strict sense. The argument is that the genesis of love is present in the mammals. Mammals also have the highest intelligence of animal life.
Most mammals certainly don't qualify as persons, but the qualities of personhood are present in most of the mammals we eat; albeit in a rudimentary and primitive form.
As I said before, none of this would be a problem if we had to eat mammals to survive or to be healthy. But we don't. There is extremely strong evidence that a vegetarian diet is not only healthy, but healthier than a carnivorous diet.
None of this of course applies to poultry. And I have no argument against eating poultry in principle. We must remember, however, the cruel treatment of animals in commercial factory farming.
The following video is typical of Factory farming (Warning there are graphic images here):
This to me is the main argument. Eating animals is not, at least for me, a serious moral failing. But the cruel, brutal, and painful way factory farms treat animals is absolutely horrifying. I think we have a moral duty not to support factory farms. Of course, one could buy all ones meat from small local farms that treat their animals with far greater compassion. In fact, this is what I do with my eggs and dairy and for this very reason. But many cannot afford this. And I still think my argument that we should avoid eating mammals because of the level of their intellect and their capability of feeling love (at least at some level) holds even for well treated mammals.
Finally, it follows from this that we cannot morally kill animals to wear them as fur or leather either. Many people have a gut reaction to all of this. They hold that we simply have a right to eat animals, to treat them however we want, and to use them for whatever purpose we see fit - no matter how cruel our treatment of them. In response I ask some questions, what are your arguments for this? On what grounds do you believe we can torture animals by forcing them into horrible living conditions? On what grounds do you bestow on human beings the right to freely abuse other creatures?