History as a field is the study of the past through written documents.
 In general, the sources of historical knowledge can be separated into three categories: what is written, what is said, and what is physically preserved, and historians often consult all three
 Since the 20th century, the study of prehistory is considered essential to avoid history's implicit exclusion of certain civilizations, such as those of Sub-Saharan Africa and pre-Columbian America

