Why I must be vegan
Most people believe animals have some kind of consciousness, but not free will. However this is necessarily irrational because it's logically impossible to have evidence for consciousness without free will, because if a creature doesn't have free will then all of its actions are explicable in terms of physical causation which means there can't be any evidence that it's conscious.
Metaphysics of consciousness
Since that belief is impossible, there are two possibilities:
1. Animals are completely unconscious, which would mean they have no rights.
2. Animals have free will, which obviously means they deserve the same rights as us.
I can't know which is true. They don't seem to act like people, but they also seem too close to be naturally occurring robots. But consider how high the probability of #2 would have to be for it to be immoral to eat animals?
If there's a 1% chance that animals are like us, then killing an animal is 1% as bad as killing a person. That's *really* fucking bad! It would obviously be completely immoral to do something that has a 1% chance of murdering someone to get you a burger.
For me the probability is about 50%, so for me, killing an animal is half as bad as murder.
I won't cover every possible argument here; I will leave that to Michael Huemer:
Dialogues on Ethical Vegetarianism