The original and traditional source of historical knowledge is the written text. However, the concept of what a historical source is has undergone transformation and redefinition over the centuries. This has happened as new mediums of communication, record keeping, and non-textual data in the form of material remains have emerged. New disciplines have also developed that continually challenge historians to include these new sources in their analysis and explanation of the past.
The primary source of history is documents, or texts in various forms. Originally historians, limited to ancient manuscripts, used a wide number of sources to achieve an understanding of the past. These sources of history came from epic poetry, myths, fables, inscriptions left on buildings or objects, deeds of farms or land, proclamations, ancient letters, and any other form of writing. Before the 20th century, historians, without the benefit of archaeology, relied on evaluating the language used, the number of copies of a text and if there was agreement, and theories about transmission of those texts to develop a means to evaluate whether a source was reliable or unreliable.