_Natural_ is a term so protean, its field of meanings so large, that it almost defies disciplining as a keyword. In the original _Keywords_, Raymond Williams makes much the same point about _nature_, calling it “perhaps the most complex word in the language.” Williams identifies three areas of meaning: an essential quality of something; a force that inheres and directs people and/or the world; and the material world, including the people within it. _Natural_ has a similarly broad range of meanings, and as with _nature_, many of these meanings still resonate today. Within the health humanities, at least four senses are discernible: natural as signifying an essential quality, relating to the material world, identifiable with human reason, and unsullied by human intervention. _Natural_ has, in the first place, been a key term in Western approaches to health, illness, and the body from Greek antiquity onward. According to the Hippocratic tradition, health is a state of dynamic equilibrium in the body, while disease is a general state of imbalance. Since disease is nature’s way of trying to restore equilibrium, healers should seek to imitate this “natural medicinal action” (_vis medicatrix naturae_) to restore health. In this task, healers have recourse to...