What's Photosynthesis Got to Do with It?
The Taxonomic Limits of Vegetarianism

I usually take the word "vegetarian" to mean someone who does not eat animals. Some people use it in a more relaxed sense to mean a person who doesn't eat certain kinds of animals, but right now I'm going to use it in the first sense. One day when eating lunch with a friend who is a vegetarian a question arose in my mind, "What's so special about animals?" An animal is a living thing, but then so is a carrot, so why is it that a vegetarian avoids eating animals but has no problem with killing and eating plants. After all, the difference between plants and animals is just taxonomy.

The Differences Between Plants and Animals

Let me define what plants and animals are, technically. I'm not a biologist, so I won't claim these definitions are authoritative, but they include the characteristics most sources seem to agree upon. An animal is a member of the kingdom animalia. Animals are multicellular organisms that are heterotrophic (meaning they survive by eating other organic material) who are capable of locomotion (meaning they can move of their own volition and don't have to be anchored in one place) at least during part of their life cycle. Animal cells do not have cell walls of the sort found in plants. A plant is a member of the kingdom plantae. Plants are multicellular autotrophs that get energy through photosynthesis and are incapable of locomotion (under their own power). Plant cells are surrounded by a cell wall. Taking all that together, the main differences between the two are that animals can move around and eat other things to live, while plants can't move (by themselves) and roll their own energy.

Reasons for Being a Vegetarian

Now we can return to the question, why is it ok to kill plants but not animals. Of course, the answer is going to depend on why you became a vegetarian. There are a lot of different reasons someone becomes a vegetarian, but often they can be placed into one of three basic categories: health, environmentalism, and ethics. My question was really focused on the last reason, but let me say a little about the other two.

Clearly, for people concerned about health, the answer could be that they just believe that it's not healthy to consume animals, but most plants are ok. That's an empirical question and a topic for another node. Let us assume for the moment that this conclusion is correct. There's not much to say here, except that, based on what is currently known, it's unlikely that there's a significant difference in health benefits between eating very little meat and eating none at all. In fact, most physicians would probably agree that while it's certainly possible to live a healthy, vegetarian lifestyle, it would probably be easier to live an equally healthy lifestyle with a small amount of meat in your diet. Then again, pragmatically, it might be easier to maintain the balance of your diet just by eliminating meat altogether, rather than trying to limit your intake.

In the case of vegetarianism for the sake of the environment, the reasoning is that raising food animals has a much larger environmental impact than raising the equivalent amount of crops. Again, this is an empirical question that I won't debate here; however, the possible objection could be raised that there may be situations in which eating particular animals does not have an unreasonably high environmental impact. There is actually an overpopulation of certain animals in some locales. So, strict vegetarianism is probably not necessary, but, again, for practical reasons it may be that strict vegetarianism is the best option (at least in the short term).

The real question I'm interested in, though, is the moral question. For those vegetarians who don't eat animals because they believe it is wrong, I want to know what is the source of the moral difference between a plant and an animal. I've mentioned the two principle characteristics that separate the a plant from an animal. Is it photosynthesis that is the capital crime of the plant, or does the motility of an animal somehow make its life more sacred? Or does it have something to do with the cell wall? Now, clearly, there are other differences like intelligence, but these are not the defining differences between a plant and an animal. Different animals have vastly different levels of intelligence, and it may not be clear that the more simple animals are any different than some plants, in terms of intelligence. An example will be helpful.

Would you, could you take the plunge? Would you, could you kill a sponge?

A sponge is a member of the phylum porifera, which is part of the animal kingdom, though you wouldn't know it to look at them. A sponge just basically sits there on the bottom of the ocean, mostly indistinguishable to the casual observer from the plants that are around it. So I wonder, would most vegetarians think it's wrong to kill a sponge?

You might wonder how it is that a sponge is considered an animal. Certainly, it's multicellular, and it's also a heterotroph. A sponge eats small creatures that swim near it and doesn't do any photosynthesis. What we normally think of as a sponge certainly doesn't seem to go anywhere, but a sponge is able to move during the early portion of its life. Once it finds a nice place to live, a sponge settles down and gets rooted to that place, not unlike many humans. What we think of as the sponge1, the kind of "natural sponge" you can buy at a store, is a sort of skeleton that the sponge organism grows once it has settled down. That's right, you've been washing yourself with a skeleton. Kinda morbid, huh?

Now, it's hard to see how a sponge can be any morally different from many plants. A sponge could easily be mistaken for a plant based on all it's external characteristics, and it doesn't seem at all clear that it shows any more intelligence than, say, a carnivorous plant. Other animals, like coral, also seem superficially quite similar to plants. Are plants really so different from animals? Cut them, do they not die? Eat them, have you not murdered?

A Fungus Among Us

Now to throw another monkey wrench into the works. People don't just eat plants and animals, sometimes they eat fungi. Although you might usually think of mushrooms as plants, members of the fungus kingdom are a sort of middle ground between plants and animals. They are organisms that cannot locomote, like plants, but they don't carry on photosynthesis and are actually heterotrophs like animals. Fungi live either as parasites or decomposers. Superficially they look much more like plants, but evolutionarily they seem to be more closely related to animals.

Knowing what I've just said about fungi, you can ask whether it's acceptable to kill them. Now, in actuality, when you eat a mushroom you usually aren't killing the fungus that the mushroom came from. In fact, the mushroom that you eat is just the reproductive organ of the fungus (the main portion of which is usually underground). Still, that reproductive organ may well be full of spores, which are essentially analogous to an animal fetus, so you're still killing off something that would otherwise become a fungus. Furthermore, even if you don't usually kill fungi to eat them, I doubt most people would think twice about killing a fungus, even among vegetarians. So, again, this seems to invite the question, what's the critical moral difference between a puppy and a portabello.

To sum up, the question that puzzles me when considering vegetarians, particularly those motivated by ethical reasons, is why they consider it unacceptable to kill animals for food but perfectly ok to eat plants and fungi. While there are, by definition, some well defined differences between those organisms, I struggle to understand which of those characteristics makes it wrong to take the life of one organism and not the other. When looking at simple animals like sponges and corals, one realizes that the differences between some animals, plants, and fungi are not as large as we might have expected. If we draw the line at taking the lives of animals, it seems necessary to ask why the line is drawn in that, seemingly arbitrary, place.

Some Possible Responses

"Because God said so!"

If your moral objection to eating animals is based upon your religion, then it may be you don't have (or feel you need) any further justification of why it's wrong. It is simply what your scripture says, and you may believe that to be infallible. That seems like a perfectly valid answer. However, in many religions there is a sort of justification given for dietary restrictions. Some religions encourage vegetarianism because of the belief that human souls may be reincarnated as animals, so killing an animal seems not too far from killing a person. One can extend the questions I've been asking and ask whether it really makes sense that a soul can be reincarnated as a sponge and not a fungus or a plant.

"You Gotta Draw the Line Somewhere"

One friend I brought this up to responded in the following way: He said essentially that he was sure it was wrong to kill a person, not sure whether it was ok to kill other mammals or fish or other complex animals, and fairly sure it was ok to kill plants. He wasn't exactly sure where the dividing line was, but by avoiding eating any animals he was fairly sure he wasn't doing anything wrong. This reasoning is based upon what philosophers might call emotive ethics, but even if you have different criteria for why you think killing a certain creature is wrong, this sort of reasoning may still apply. Generally, you could respond that drawing the line of where killing is acceptable at the edge of the animal kingdom is arbitrary, but this is because the actual line is somewhere inside. The line is drawn at animals because this is more practical, for whatever reason. In my friend's case, the reason was uncertainty. Of course, we then must ask what the actual criteria are and how you know the real dividing line lies within the animal kingdom.

"Hey, you're right, salad is murder!"

There are some people who believe that killing plants is wrong, because, as living creatures, they don't deserve to be murdered any more than animals. Such people may become fruitarians, eating only those parts of organisms that don't involve killing the organism itself, like the fruit of a plant. You might not go quite that far and could take the pragmatic position that killing plants is wrong, but you have to eat somehow. Given that fruitarianism is difficult at best, you could try to eat a diet in which you try to minimize (but not reduce to zero) the number (or amount measured in some other way) of living organisms you kill for subsistence. This would still leave you with the task of determining which lives are more acceptable to take and how to minimize the damage you do by existing.

"Fine, eat 'em all, let God sort 'em out!"

Clearly one response would be to simply give up on the idea of vegetarianism and go back to eating all those cuddly animals. The idea being that there's no meaningful moral difference between plants and animals, so if it's ok to eat plants it must be ok to eat animals. Of course, if you take this position then you must go back and try to answer the basic question of the vegetarian, "Why is it ok to kill other animals but not people?" Again, this is a topic for another node and is not necessarily an easy question to answer.

1 Here I'm talking about what's normally called a "natural sponge". More than likely, what you think of when you think of a sponge is the kind you might use for cleaning your dishes or your house, which is actually made out of cellulose (thus probably being vegan) and is only made to mimic a natural sponge. Gorgonzola reminds me there are also loofahs, which are plant matter but also sort of like a skeleton. They come from a gourd.


  • Wikipedia, articles on animal, plant, and fungi
  • Various E2 nodes linked within
  • Dim recollection of high school biology