Most arguments about it being wrong to eat animals boil down to the pain and suffering we cause animals in order to raise and slaughter them.
Our understanding of pain and suffering is based on the particular kind of pain and suffering we experience. This experience is made possible by our possession of a complex central nervous system (CNS). Animals also have CNSs, therefore they experience pain and suffering too.
Plants, on the other hand, have no CNSs. But they do have organs that transmit messages both within and between individuals. They also have sensory capabilities that are surprisingly sophisticated: they know up from down, they can determine the nutritional content of the surrounding soil, they can move towards light, and so on.
I will abbreviate these organs and sensory capabilities as the veggie nervous system, or VNS.
Pain and suffering is a phenomenon, not an observable physical characteristic. It’s like a software program running on hardware: you can no more cut open a brain and find the pain and suffering in it than you can break open a computer’s hard drive and find a spreadsheet or a video file.
If we experience complex human-CNS pain and suffering, then a cow or pig experiences cow- or pig-CNS pain and suffering. It stands to reason that plants also experience plant-VNS pain and suffering. Simple hardware still runs software, just a simpler kind.
Now consider all we do to plants, even just in subsistence farming (let alone industrial farming), landscaping, and in food preparation. It’s ghastly even by the standards of the meat industry.
We are therefore faced with two alternatives: either it’s morally wrong to eat virtually anything that didn’t just fall off a tree, or the morality of eating certain kinds of organisms is more of a case-by-case judgment call and generalizations about eating members of whole kingdoms don’t really work. Ahisma Jainists seem to acknowledge the existence of this dilemma, and they have chosen some version of the “eat only fallen fruit” horn. Just about everyone else chooses the latter “eat whatever after a case-by-case judgment call” horn.
Only vegans and moral vegetarians are stuck in the middle, arguing against one horn while ignoring or unaware of the implications of the other. Fortunately for them, they don’t starve there.