What is and isn't considered historical evidence is not a very controversial question. It is widely excepted that primary sources are the most reliable evidence and secondary sources are second in reliability. Primary sources are: is a document or physical object which was written or created during the time under study. These sources were present during an experience or time period and offer an inside view of a particular event. Secondary sources analyze and interpret primary sources. They are one or more steps removed from the subject at hand.

Historians also examine aspects of the sources such as origin, purpose and intentions when attempting to determine what is reliable facts. All evidence has some sort of bias, weather it is conscious or subconsciously, and this needs to be taken into consideration.