We can think of the world, for any person, as divided into the self and everything else. The principal material breach of this fundamental dichotomy occurs in the act of ingestion, when something from the world (other) enters the body (self). The mouth is the guardian of the body, a final checkpoint, at which the decision is made to expel or ingest a food.

There is a widespread belief in traditional cultures that “you are what you eat.” That is, people take on the properties of what they eat: Eating a brave animal makes one brave, or eating an animal with good eyesight improves one’s own eyesight (reviewed in Nemeroff and Rozin 1989). “You are what you eat” seems to be “believed” at an implicit level, even among educated people in Western culture (Nemeroff and Rozin 1989). It is an eminently reasonable belief, since combinations of two entities (in this case, person and food) usually display properties of both. Thus, from the psychological side, the act of eating is fraught with affect; one is rarely neutral about what goes in one’s mouth. Some of our greatest pleasures and our greatest fears have to do with what we eat.

The powerful effect associated with eating has a strong biological basis. Humans, like rats, cockroaches, raccoons, herring gulls, and other broadly omnivorous species, can thrive in a wide range of environments because they discover nutrients in many sources. But although the world is filled with sources of nutrition, there are two problems facing the omnivore (or generalist). One is that many potential foods contain toxins. A second is that most available (nonanimal) foods are nutritionally incomplete. An apt selection of a variety of different plant foods is required for the survival of omnivorous animals to the extent that they cannot find sufficient animal foods. Animal foods tend to be complete sources of nutrition, but they are harder to come by because they are less prevalent and because they are often hard to procure (for example, they move). Hence, the omnivore must make a careful selection of foods, avoiding high levels of toxins and ingesting a full range of nutrients. Any act of ingestion, especially of a new potential food, is laden with ambivalence: It could be a good source of nutrition, but it might also be toxic.