miss_chris:

Hey Ghost, The far-from-arbitrary line we draw is a little thing called a nervous system and pain receptors. When you approach a plant with a hot branding iron, it doesn't recoil in fear. When you place the hot branding iron on the plant, it doesn't scream out in pain, it doesn't try to get away. Animals are thinking, feeling beings that feel every single bit of horrific pain we cause.

I know that this is essentially Peter Singer's argument in Animal Liberation (which I have read). But I find there to be several problem with this argument. First of all, the argument only seems to go so far as to say that our moral obligation is to not cause pain in other animals. It is not the killing or eating of the animal that is wrong per se, but the pain that precedes its death. It leaves open, then, the possibility that consuming the animal is morally defensible so long as it was given a painless death (and by surprise as well, perhaps).

Secondly, pain receptors, fear reactions, screaming, etc., are, in a biological sense, defense mechanisms. Just because a vertebrate's defense mechanisms work through a certain aparatus (i.e. the central nervous system), does not magically imbue that aparatus with moral worth. The purpose of pain is to warn the animal that it is or is being injured so that it can recognize the stimulus of the pain and respond defensively. The animal then stores the stimulus-pain association in its brain. When the animal encounters the stimulus again, it recalls this association and can react (what we might call "fear") before it gets to physical contact. Screaming can serve to frighten predators or to call for help. Plants and other animals also have defense mechanisms for the times in which they are threatened. Pain is, of course, unpleasant, but the evolutionary reason is obvious--so that the animal reacts to it quickly (leprosy is no enhancement). Evolution is like an arms race. Creatures have evolved by adding new defenses or new weaponry. (disclosure: I have serious doubts concerning evolution; I use it only for the sake of argument.) The sophistication of your weaponry or defensive apparatus does not automatically give you moral superiority. It is folly to argue morality from biological differences (think: eugenics).

Thirdly, it is a huge leap to go from "pain is unpleasant" to "I ought not cause others pain." A preferance for the avoidance of pain does not, in the next step, become a moral imperative to refrain from inflicting it on others. What you are attempting to do is construct the Golden Rule from some other first principle, but it is the first principle; it is the precedent, not the antecedent. Because extending the Golden Rule to all living things is impractical--nay, impossible--a feeble attempt is made to limit its scope by supposing that it is inclusive of only vertebrates.

You also mention "thinking" and "feeling." These too are biologically based acts that aid in the survival of the animal, allowing it to adapt to its environment. The sophistication with which a creature adapts to its environment is not directly proportional to the creature's moral worth. The two have nothing to do with one another. The line you draw is therefore arbitrary because it rests on choosing creatures eligible for moral worth based soley on the sophistication of their biology.

Finally, the universe is cold and unfeeling, and nature is brutal and unforgiving. If you are trying to extract first principles of morality from it, you are looking in the wrong place.