Rabbi HaCohen Kook’s essay on vegetarianism and peace, first published in instalments in 1903–4, and reissued 60 years later, is the only treatise in rabbinic Judaism on the relationship between humans and animals. It is here examined as central to his ethical beliefs. His writings, shaped by his background as rabbi and mystic, illuminate the history of environmental and applied ethics. A century ago, he perceived the main challenge that confronts reform movements: multiculturalism.