Answer:
History is made with sources. A source is that which allows us to verify a historical fact. The traditional conception of history only considers the written text as a source. In fact the history of two parts is divided: Prehistory, before the written texts, and History, from the appearance of the writing. But, obviously, this is a reductionist vision of history. History is everything that happens since the appearance of humanity on Earth, whether there is text or not. The Annales school will deny the written document as the undisputed and maximum source of historical knowledge. Any realization that starts from human activity will be a source.
Sources, then, are considered to be written texts, archeology, oral traditions, art, etc. Everything that humanity has produced and everything that can give us information about the past. Evidently, the written sources are the most used: stories, letters, statistics, literature, registers, files, toponymy, etc. The document par excellence is the legal text, but this is interested, since it leaves evidence of what matters to who has the power. A History book is not a historical source, since it is a book that already interprets the documents; although it can be a source for the History of historiography.
The biggest problem the historian faces is how to know the facts of the past. For this we must look for testimonies that tell us, sources that are often scattered. But once located can not be believed without more, it is necessary to verify its authenticity, its veracity, what in the document is ornament, if there are concealments, etc. The preserved documents do not speak of what interests us, but of what interests those who make them. Therefore, certain facts can be very few documents, while others exist countless data, which is necessary to make a process of selecting the documents, the Heuristic.
The fundamental problem is to determine the degree of reliability of the source, especially if it is written. You have to determine its authenticity, know its origin, know the degree of credibility and make a critique of it. But, once the source has been studied and understood, it is not enough to publish the fact, it is necessary to interpret it and elaborate historical knowledge.
