sensei wrote: Many people consider themselves to be vegetarian, yet also eat fish every day.
A reason why fish isn't (always) considered as meat goes back to the christian/catholic religion. Besides the story that Jesus presented his disciples fish, the Catharsers living in the south of France in the Dark Middle Ages added some philosophical ideas, being the main reason in this context that fish is no real meat because it's white meat (...).
In the current western civilization it is more or less accepted that the brain is the main thing of the body, but the Catharsers thought it was blood, and by eating blood of animals (as if humans aren't animals...), you would assimilate the vibrations and "essence" of that lower animal. Aka: you are what you eat. They thought that white meat like fish and chicken doesn't contain blood, hence would not take your spirit/soul down/degenerate to the animal level. Also, when eating vegetables and white meat etc. it was supposed to take you up to the highers spheres of enlightenment and the good old days of eden and the likes. The latter is stolen from the greek mythology; more precisely, the times before the gods and goddess: the time of Kronos, where there was peace in the world and plenty of food so that there was no need to kill animals and eat meat: "the heaven on earth".

Well, that's what they thought. Btw, there is no statistical correlation that eating meat results in more violence, only that there may be a tendency that vegetarians, in general, tend to be less violent. An interesting book that discusses some of these ideas is "The Heretic's Feast : A History of Vegetarianism" by Colin Spencer.