Vegetarianism, the theory or practice of living solely upon vegetables, fruits, grains, legumes, and nutswith or without the addition of milk products and eggsgenerally for ethical, ascetic, environmental, or nutritional reasons. All forms of flesh (meat, fowl, and seafood) are excluded from all vegetarian diets, but many vegetarians use milk and milk products; those in the West usually eat eggs also, but most vegetarians in India exclude them, as did those in the Mediterranean lands in Classical times. Vegetarians who exclude animal products altogether (and likewise avoid animal-derived products such as leather, silk, honey, and wool) are known as vegans. Those who use milk products are sometimes called lacto-vegetarians, and those who use eggs as well are called lacto-ovo vegetarians. Among some agricultural peoples, flesh eating has been infrequent except among the privileged classes; such people have rather misleadingly been called vegetarians.

Deliberate avoidance of flesh eating probably first appeared sporadically in ritual connections, either as a temporary purification or as qualification for a priestly function. Advocacy of a regular fleshless diet began about the middle of the 1st millennium bce in India and the eastern Mediterranean as part of the philosophical awakening of the time. In the Mediterranean, avoidance of flesh eating is first recorded as a teaching of the philosopher Pythagoras of Samos (c. 530 bce), who alleged the kinship of all animals as one basis for human benevolence toward other creatures. From Plato onward many pagan philosophers (e.g., Epicurus and Plutarch), especially the Neoplatonists, recommended a fleshless diet; the idea carried with it condemnation of bloody sacrifices in worship and was often associated with belief in the reincarnation of souls and, more generally, with a search for principles of cosmic harmony in accord with which human beings could live. In India, followers of Buddhism and Jainism refused on ethical and ascetic grounds to kill animals for food. Human beings, they believed, should not inflict harm on any sentient creature. This principle was soon taken up in Brahmanism and, later, Hinduism and was applied especially to the cow. As in Mediterranean thought, the idea carried with it condemnation of bloody sacrifices and was often associated with principles of cosmic harmony.

In later centuries the history of vegetarianism in the Indic and Mediterranean regions diverged significantly. In India itself, though Buddhism gradually declined, the ideal of harmlessness (ahimsa), with its corollary of a fleshless diet, spread steadily in the 1st millennium ce until many of the upper castes, and even some of the lower, had adopted it. Beyond India it was carried, with Buddhism, northward and eastward as far as China and Japan. In some countries, fish were included in an otherwise fleshless diet.

West of the Indus the great monotheistic traditions were less favourable to vegetarianism. The Hebrew Bible, however, records the belief that in paradise the earliest human beings had not eaten flesh. Ascetic Jewish groups and some early Christian leaders disapproved of flesh eating as gluttonous, cruel, and expensive. Some Christian monastic orders ruled out flesh eating, and its avoidance has been a penance and a spiritual exercise even for laypersons. A number of saints, such as St. Anthony of Egypt, were noted vegetarians. Many Muslims have been hostile to vegetarianism, yet some Muslim Sufi mystics recommended a meatless diet for spiritual seekers.

The 17th and 18th centuries in Europe were characterized by a greater interest in humanitarianism and the idea of moral progress, and sensitivity to animal suffering was accordingly revived. Certain Protestant groups came to adopt a fleshless diet as part of the goal of leading a perfectly sinless life. Persons of diverse philosophical views advocated vegetarianism; for example, Voltaire praised it, and Percy Bysshe Shelley and Henry David Thoreau practiced the diet. In the late 18th century the utilitarian philosopher Jeremy Bentham asserted that the suffering of animals, like the suffering of humans, was worthy of moral consideration, and he regarded cruelty to animals as analogous to racism.

Vegetarians of the early 19th century usually condemned the use of alcohol as well as flesh and appealed as much to nutritional advantages as to ethical sensibilities. As before, vegetarianism tended to be combined with other efforts toward a humane and cosmically harmonious way of life. Although the vegetarian movement as a whole was always carried forward by ethically inclined individuals, special institutions grew up to express vegetarian concerns as such. The first vegetarian society was formed in England in 1847 by the Bible Christian sect, and the International Vegetarian Union was founded tentatively in 1889 and more enduringly in 1908.

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