I can remember that, over a period of time, I came to feel that there was something not quite right about eating meat. I couldn't really put it into words; it was just a vague uneasyness. But what made it come together was when my dog was hit by a car. It happened at dinner time, and to this day I remember how they brought him in and laid him in the kitchen, waiting for someone (I don't remember who) to come and take him to the vet. And I remember looking at my roast beef, and losing my appetite. I couldn't have told you then, but now I know that somewhere in my brain I realized that the roast beef on my plate was not so very different from the injured flesh of the dog I loved. Here I was, crying over the injury of one animal; how could I possibly choose to kill some other?
The dog, by the way, made a full recovery.
Many years have passed since then, and being the analytical sort of person that I am, during that time I've tried work out in rigorous, logical terms, exactly why it is that I feel as I do about the ethical consideration that is due animals. It's always difficult to make an ethical argument. An ethical argument has to be based on some fundamental ethical principals or axioms; if someone else doesn't accept those principals, no amount of argument will sway them.
What we can look for, however, is consistency. It seems to me that it is somewhat irrational to one hold act as, if you will, "wrong", and another fundamentally similar act to be not "wrong." I also hold that I do not wish to be harmed. (Again, we generally consider those who wish themselves harm to be irrational). Now, I observe that other humans are fundamentally similar to me. (More on this in a minute). Therefore, an act which (unnecessarily) harms another human is fundamentally similar to an act which harms me. I therefore judge such an act as "wrong".
What do I mean by "fundamentally similar?" Is is just appearance? No. I do not find the destruction of a mannikin to be in any way evil. Is it sameness of behavior? No; people of other cultures may have behavior that is radically different than my own. Or to take an extreme case, a person afflicted with severe mental retardation exhibits behavior that is almost nothing like my own. Yet I would find anything that harms such people to be "wrong".
Is it genetic similarity that makes for this similarity? No; if it were, I would feel free to make ethical distinctions based on racial heritage. What I'm left with, then, to consider for this fundamental similarity is is the similarity in our minds - or, more precisely, in the functioning of our brains. Knowing that my brain is what experiences my thoughts and sensations, and knowing that other humans have similar brains, I conclude that other humans have similar experience of thoughts and sensations.
Ok. What does this have to do with vegetarianism? Other vertebrates, especially mammals, have brains that are fundamentally similar to those of humans, in form and in function. (I should note that it's function that concerns me more - a conscious computer, or a extraterrestrial with completely different physiology, should receive ethical consideration independent of the physical strata of their minds.) We can also observe this similarity in the behavior of such animals. I therefore hold that it is wrong to unnecessarily kill these "higher" animals. Now, as the similarity weakens, so does my concern. I feel only a small twinge at smashing a roach, or swatting a fly. (So I ain't the Dali Lama.) But there's no way I'll kill a fellow animal for food or skins, or imprison and mistreat one for its eggs or milk. Their suffering is too much like my own.
A lot has changed since I made my choice. Ten years ago, the response to "I'm a vegetarian," would be a funny look and a smart, insulting remark. Today, the response is more likely to be, "Gee, I'd like to know more about that." If that sounds like you, here's your chance. Enjoy!