How about this, can you guarantee that not every living thing experiences pain in some way? Is a vertebrate nervous (or descendent of bilateria) system the only mechanism or can all living things respond to threatening stressors according to their own way?

Or even more general, and more important than just pain, can you guarantee that every living thing doesn't "experience" the universe according to their own way? Perhaps in ways beyond our ken?

(I think the general experiential test is more important than pain, things that don't feel pain shouldn't be invalidated because of that. Otherwise people who have no pain would be valueless, which I think we can agree isn't right either).

If a living thing wasn't gifted with neurons and fast response muscle fibers then is it fair to draw the line such that "only things gifted in experiencing the universe like I do are off limits because those are the only valuable experiences worthy of life?"

For example, we know plants can "move" (twisting and turning of leaves, growing of branches and root) to get more sunlight and soil nutrients. And plants can respond to long-term and near term threats with a variety of short and long-term responses...even sacrificing themselves in some cases to cast off the literal seeds of the next generation if need be.

Some plants have even built carnivorous mechanisms so smart they can tell the difference between dietary animals in its diet and animals not in its diet -- most dogs can't even figure this out and will gobble down all manner of things. These plants have no recognizable nervous systems and we're not even quite sure how they experience the universe such that they can do this, and then move faster and stronger than their prey in order to eat. Can you promise me that plants don't feel something? Even in a way, and using mechanisms and cognition, different than our own?

We know plants can smell, see, remember, communicate with each other, can signal different parts of itself, have memories (all of these of course according to the ways of a plant) and other hallmarks of "higher-level" organisms. Can you promise me that among these things, a plant doesn't hurt when it's turned into an ingredient in a salad, or baked in an oven or sautéed.

Single cell organisms even show many of the fight or flight responses of larger, more, complex, more neurologically gifted organisms. They'll hunt out and find nutrients, flee from threats, some will create and release stress molecules when under attack. "It's just chemistry" just insults the unique nature of living things. You're just chemistry also. Just because your ancestors developed a more complex Rube Goldberg machine for threat response and finding nutrients doesn't mean you should diminish those living things less fortunate than you. Can you absolutely guarantee that microorganisms don't experience "pain" according to their own way?

We know different kinds of living things work in different kinds of ways. So different it's hard to even come up with a simple definition of "life". Is it any surprise that different living things have different kinds of experiences and ways of "computing" those experiences?

Plants have glutamate receptors (analogous to higher organism's system for memory formation and learning). Sea Sponges, just like plants, have no neurons at all, but can use various mechanisms for whole body contraction and other intra-cellular communication -- like calcium waves. Radiata, some of the simplest multi-cellular mobile animals, have nervous systems, but some threat responses, like stingers, are wholly automated and disconnected from their nervous systems entirely.

You don't know if a cow or a pig or a chicken experiences pain. They could just be operating according to an evolutionary pre-programmed instinctual chemical trigger response that you are simply personifying as pain. The reaction of a cow to a threat is not much different than a bacteria to a macrophage.

Let's suppose an Alien drops to Earth tomorrow. But because of how their species evolved experiences the universe in ways we can't even begin to imagine. Should we invalidate its existence and claim it as a possible food simple because of this?

Do you draw the line only at "things with nerves" or "things with an identifiable central nervous cluster" or "things that move on timescales I can recognize?" Or "things that have pain responses I can recognize?" or "things that have pain responses I can personify?"

Is your diet okay simply because we're ignorant of the alternate experiences of other kinds of life, or did you just draw the line there because that's where you stopped feeling bad?

Is ignorance and lack of imagination a valid way to draw moral thresholds? I don't know if a plant hurts when I eat it, therefore it's okay?