why does thoreau believe that to preserve ones poetic faculty (higher nature) one must abstain from animal food

Most men, if they had to prepare it themselves, would give up eating rich cooking. Man's carnivorous instincts are accomplished in a "miserable way" through the slaughter of animals. Thoreau is certain that it is the destiny of the human race to stop eating animals, just a tribes gave up cannibalism when they became civilized. Men should follow their inner genius, even if it means feeling bodily weak, to conform with higher principles, and they will be rewarded by life becoming "more elastic, more starry, more immortal."

Thoreau's metaphor of the body as material for the artist juxtaposes strangely with his primarily symbolic perspective. In likening the sculptor's creation of art from clay to man's creation of art from his body, he denigrates the symbolic nature of the sculptor's work. This is odd, primarily because Thoreau himself is so heavily concerned with symbols. For example, his refusal to eat meat, coffee or tea, or salt and spices is primarily a symbolic statement. Vegetarianism, for him, is not an end in itself. A Puritan eating brown bread can be a glutton if he gives in to his base physical desire for it. Additionally, because Thoreau allies himself with animals as coexistent parts of nature, to eat them would be tantamount to cannibalism.