The shortest and simplest version of the argument I can think of is this:
Premise a): There is an ethical responsibility to avoid inflicting unnecessary suffering.
Premise b): In a western society, survival is not dependent on the consumption of animal products.
Conclusion/Premise c): All suffering that arises from the consumption of animal products is unnecessary suffering.
Conclusion: There is an ethical responsibility to avoid consuming animal products.
So far, so good. But I've had this discussion often enough to know that few people will leave it at that. By now I have a pretty good clue what's coming, so let me pre-empt a few of the obligatory defenses for meat-consumption that people will come up with:
The Naturalist's Argument:
Humans have been eating meat for thousands of years. Eating meat is our nature.Humans are omnivores. We're able to digest just about anything, and that means our diet is our choice. And where there is choice, ethics applies. Also, you might wanna look up 'appeal to nature' - You'll find that it's a logical fallacy.
The Localist's Argument:
Factory-Farming's bad, but eating locally grown meat can't be wrong. (In other words: "How about we're just really nice to our cows before we slaughter them?")You'd still kill them. Shockingly, that still qualifies as an infliction of unnecessary suffering. Of course it's better than steak from factory-farms, but it's still suffering that is inflicted on conscious beings for no other reason than that you enjoy the taste.
I'd further like to point out that by the same line of reasoning, a parent can justify eating their kid, as long as they loved him and treated him well until they decided to end his life because 12-year-olds taste just so damn good.
The Argument from Tigers:
But other animals eat meat, too! You wouldn't judge a tiger for eating meat!That's right: I do not judge a tiger for its diet, for the same reason I don't judge a rock for falling on somebody's head. The tiger has no say in the matter. The key word here is choice, because only where there is choice there is the responsibility to choose wisely. A tiger can't help itself. You, on the other hand, you decide whether to buy a factory-farmed steak or rather eat a vegetable stew instead. Your infliction of suffering is a choice, and thus you're responsible for making it.
The Hypothetical-Plant-Suffering Argument:
You know, plants react to their surroundings, too! Not to mention all the insects and other tiny animals that are killed in the process of harvesting all those plants. As long as your diet isn't free from suffering, how dare you judge me for eating the flesh of dead baby cows?Let's just ignore for a moment that most plant's fabled reactions to their surroundings are purely mechanical and/or chemical in nature, not unlike a flower-pot displaying the "reaction" of falling down after being pushed over an edge. Even if the claim that plants suffer is taken at face value, it overlooks the very basic fact that animals, too, require food.
So for each and every unit of vegetarian diet I consume, a meat-eater's food has eaten several times that amount of vegetarian diet prior to its killing. In the end, basic maths destroys the argument.
Eating meat is just a terribly inefficient way to process a plant diet: A cow consumes 6kg of grain to produce single kilogram of meat. So whatever amounts of plant matter I consume and whatever amount of suffering is entailed in its production, it takes at least six times that amount to feed the animal which will eventually end up as a steak on your plate. It is also worth noting that I used the words "at least" because a 6:1 grain-to-meat-conversion ratio is the most careful estimate I could find. Other estimates, taking into account the energy consumed by the transport, animal sheltering and other intermediate steps in the production of meat, run as high as 23:1.
So in order to debunk the argument I do not even need to engage the highly ridiculous notion that plants, despite their lack of nervous systems, are capable of "suffering" in any meaningful sense of the term. The question is moot, because whatever suffering there is entailed in a veggie diet, any meat diet entails that amount multiplied by six, plus the suffering of the animal itself.
To put it another way: If your argument is that both of us consume living organisms, my answer is that I consume living organisms, while you consume living organisms who consume six times as many living organisms as I do.