Not three same person, but the demarcation between what should be OK to eat, and what - not - they baked most sense to me, is the capacity to experience pain or emotions.
So I see no substantial moral difference between eating plants or invertebrates, for example - neither can feel harm.
That said, fish and chicken can experience pain or emotions the same as cows and pigs.
OK. They have a brain to feel them with. If you're objecting to imprecise terminology here, I'll give you the point, but I don't think that affects my basic point any (I'm not a biologist, I meant insects and the like - though don't take that as definitive either; maybe someone knows an insect with a brain, too).