A historian is an individual who studies and writes about history, and is regarded as an authority on it. Historians are concerned with the continuous, methodical narrative and research of past events as relating to the human race; as well as the study of all events in time. If the individual is concerned with events preceding written history, the individual is a historian of prehistory. Although "historian" can be used to describe amateur and professional historians alike, it is reserved more recently for those who have acquired graduate degrees in the discipline. Some historians, though, are recognized by equivalent training and experience in the field. "Historian" became a professional occupation in the late nineteenth century at roughly the same time that physicians also set standards for who could enter the field.