First of all, I think we should probably all be vegetarians. The argument goes like this. Suppose that we're visited by aliens from outer space, who are significantly more intelligent and powerful than we are, and regard us as animals. They decide to start systematically farming, killing, and eating us. How could we regard them as wrong, or criticize their behaviour? It's merely what we've always done to others.
In practice, I think it's rather unlikely that aliens from outer space would find humans either tasty or nutritious, but it's possible, so we should take it into account.
So far, I haven't thought of any good answer to this argument, and I suspect that humanity will gradually turn vegetarian in the future. However, I list below an assortment of my personal excuses for not yet becoming a vegetarian myself.
- If the aliens turn up, they'll observe that humanity in general farms, kills, and eats animals. I don't suppose they'll distinguish between one human and another. So going vegetarian at this point wouldn't do me any good.
- Having grown up in an omnivorous society, it's what I'm accustomed to, and it's the way society encourages me to live. Going against the grain of society is difficult in various ways.
- If there were no humans on the planet, animals would rarely be able to die peacefully in their sleep. I think the normal ways would be to die painfully at the teeth and claws of some other animal, to die of disease, or to die of starvation when unable to get enough food. By killing and eating animals, humans aren't really introducing anything new to the situation: animals eat each other and most of them die painfully, one way or another.
- On an ideal farm, animals may actually live better lives than in the wild. They're looked after, they get food and shelter in winter, their illnesses are treated, they're not usually attacked by carnivores, and in the end they're killed humanely. On a modern intensive farm, life probably isn't worth living, and they may die unpleasantly too; but that's the choice of the farmer. It can be done either way.
- I'd be happy if all farms were ideal farms, although meat would then be much more expensive and most people would eat it only occasionally (which would probably be good for them). If farmers mistreat animals, that's on their conscience. It's not on my conscience, because I neither do it myself, nor do I force them to do it. If I buy their products, I give them a tiny encouragement, but it's so tiny as to be negligible. Whether I buy meat or not is not going to make any difference to any farmer's decisions.
- In principle I should seek out ideal farms and buy only from them, but even if this is feasible it would involve considerable time, effort, and expense, and I have to balance it against the negligible practical effect that my efforts would have.
- These days my wife buys the food, anyway!