History is the systematic study and documentation of the human past. It is an academic discipline that uses a narrative to describe, examine, question, and analyze past events, and investigate their patterns of cause and effect. Historians seek knowledge of the past using historical sources such as written documents, oral accounts, art and material artifacts, and ecological markers. History covers all aspects of human society, including political, social, and economic changes. It is the study of change over time. Historians emphasize the value of primary sources, which are sources actually dating from a particular time period, while understanding the limitations of such sources. Non-historians often make broad generalizations about people, ideas, events, or time periods in history.