The best argument against vegans is always the fact that plants also are living beings. Now if you are gonna create hierarchy of living beings to justify your food consumption, well...
Cows don’t photosynthesize they eat a shit ton of plants to make a tiny amount of meat so if you really care about plants you would eat the plants directly and skip the middlemen that waste 90% of the plant matter
Plants aren't sentient though, that's a pretty good reason to put them lower on the hierarchy of living beings that are morally ok to eat. And it's quite likely that fewer plants die for a vegan diet than for a standard diet, as animals need a lot of feed to produce meat, eggs and dairy. Some percentage of the plant protein, fats, and carbs will always be lost along the way when we feed them to animals, so eating those plants directly is more efficient.
Sentience is hard to measure though. Also I had a weird discussion with my neighbour once who argued that in order to kill fewer sentient beings, we should eat the bigger ones as the ratio of meat per sentience was better, so we should really eat whales. Which made it pretty obvious to me that a) he was nuts and b) sentience might not be the best indicator for ethic food consumption.
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That doesn't mean that I oppose the idea that eating plants is better. I'm just arguing against sentience as a good indicator.
Except his argument is flawed on its own grounds, because the bigger the animal the more food it takes to support it before you come along and kill it. Assuming we had an objective measure of sentience, it's pretty likely most non-herbivores are costing more sentience than your save by eating them.
At which point we should just cause the extinction of all animals except humans and the few plants needed to support humans. See how that's a horrible metric?
And how does that prove anything? Can you prove sentience requires neurons? The clear answer is no, that's just a confortable assumption we make to not be shackled to phylosophical inertness.
What do you think they have to feed to the animals? If you believe plants to be important enough, you should go vegan to reduce their suffering. Also, do you get really annoyed at people walking on grass? How about if you see someone kick a dog? I swear, when it comes to the veganism devate, normally sensible people completely lose all sense of logic.