When we think about diagnosis, most of us think about a sick person going to the health-care provider with a collection of signs and symptoms of illness. The provider, perhaps with the help of some tests, names the disease and tells the patient if and how it can be treated. The cognitive process of diagnosis involves integrating information from history, observation, exam, and testing using a combination of knowledge, experience, pattern recognition, and intuition to refine the possibilities. The key element of diagnosis is assigning a name to the patient’s illness, not necessarily deciding about treatment. Just as we name a recognizably distinct animal, vegetable, or mineral, we name a recognizably distinct disease, so we can talk about it and study it.