A basic fact of learning is that people quickly learn new associations without rapidly forgetting old associations. Presumably this ability is highly adaptive for any creature that confronts a rich and complex environment. Consider a hypothetical situation in which an animal learns that mushrooms with a round top and smooth texture are tasty and nutritious. After successfully using this knowledge for some time, the animal encounters a new mushroom with a smooth texture but a ﬂat top. This mushroom turns out to induce nausea. How is the animal to quickly learn about this new kind of mushroom, without destroying still-useful knowledge about the old kind of mushroom? If the animal learns to associate both features of the new mushroom with nausea, then it will inappropriately destroy part of its previous knowledge about healthy mushrooms, i.e. the previous association from smooth texture to edibility will be destroyed. On the other hand, if the old association is retained, it generates a conﬂicting response, i.e. eating the mushroom.