There has been yet another mass shooting in the United States. This one is the deadliest in living memory. In the wake of this horror, people are having the usual arguments about whether the shooting was an act of terrorism and whether the shooter was mentally ill. The first is a legal question, and the second is a medical question. To answer the first question, you need to understand how legal terms are defined. To answer the second, you need to understand what mental illness is and how cases of mental illness are classified. These questions are important. Until you answer them, you cannot figure out how to prevent similar offenses from happening in the future.