Sentience refers to the ability of an entity to have subjective perceptual experiences. It is distinct from consciousness, which covers the mind and thought. Why is this important?
In animal rights philosophy, sentience means that animals have the ability to experience pleasure and pain. They feel pain, therefore killing and eating them is wrong. The problem is that scientists consider plants to be sentient too, since they can feel, touch, taste, smell, and respond to their environments.
Indeed, the biochemical pathways are different, but that's true of every living thing: a daffodil is not quite a venus fly trap is not quite a honey bee is not quite a goldfish is not quite an octopus is not quite alligator is not quite a monkey is not quite a human. All life (plant and animal) varies in fundamental biochemistry. The point is that plants and animals have fundamental biochemistry, and are sentient. If an animal is sentient, wrong to hurt; if a plant is sentient, wrong to hurt. Clearly, under this ethical model, we have nothing to eat.
Some vegans argue that plants are not sentient, but that would never fly with a scientist. Smarter vegans argue that plants are sentient but still unconscious, which is true, but takes the issue away from sensation and into complex neurological thought, which is just an attempt to confuse the topic. The best vegan rebuttal I found:
Even if it’s true that plants are the most sentient life on Earth, veganism would still be the minimum standard of decency. This follows from the simple fact that animals are reverse protein factories, consuming multiple times the protein in plant food that they produce in protein from their flesh and bodily fluids.
Of course, this is nonsense on multiple levels. First, animals are not just made out of protein. Second, a harmonious biotic community requires inconsistent "factories" to maintain the circle of life. Third, if animals are 'wasting' the essence of life, then the ethical thing would be to kill the animals. Clearly, this argument is flawed.
As for the idea that plants are less sentient than animals? This statement would be due to identification bias; in essence, we confuse their consciousness with their sentience and conclude that plants don't appear to be as sentient as we are. But looks can be deceiving. According to the Human Genome Project, we have about 25,000 genes -- the rice grain has twice that. Truth is: as lifeforms, we didn't evolve more. We evolved differently. Plant species exhibit vast diversity and powerful sophistication in response to and engagement with their natural environments. They can smell smoke, see sunlight, and -- you guessed it -- feel a limb get ripped off.
Pain and sentience are incomplete arenas to conduct a moral discourse. We need big picture morality, one that takes into account the health of the biotic community, weighs opportunity costs, and questions underlying assumptions.
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