Roman writer Ovid concluded his magnum opus Metamorphoses, in part, with the impassioned argument (uttered by the character of Pythagoras) that in order for humanity to change, or metamorphose, into a better, more harmonious species, it must strive towards more humane tendencies. He cited vegetarianism as the crucial decision in this metamorphosis, explaining his belief that human life and animal life are so entwined that to kill an animal is virtually the same as killing a fellow human.
Various ethical reasons have been suggested for choosing vegetarianism, usually predicated on the interests of non-human animals. In many societies, controversy and debate have arisen over the ethics of eating animals. Some people, while not vegetarians, refuse to eat the flesh of certain animals due to cultural taboo, such as cats, dogs, horses or rabbits. Others support meat eating for scientific, nutritional and cultural reasons, including religious ones. Some meat eaters abstain from the meat of animals reared in particular ways, such as factory farms, or avoid certain meats, such as veal or foie gras. Some people follow vegetarian or vegan diets not because of moral concerns involving the raising or consumption of animals in general, but because of concerns about the specific treatment and practises involved in the raising and slaughter of animals, i.e. factory farming and the industrialisation of animal slaughter. Others still avoid meat because meat production is claimed to place a greater burden on the environment than production of an equivalent amount of plant protein.
The simplest definition of vegetarianism is a diet free of meat, fish, and fowl flesh. But eating habits of vegetarians cover a wide spectrum. At one end are lacto-ovo vegetarians, who avoid animal flesh but eat eggs and milk products. At the other end are vegans, who forgo eating (and often wearing) all animal-based products, including honey. Raw foodists are vegans who eat mainly raw fruits, vegetables, legumes, sprouts, and nuts.