The Ghaznavid historian Abū al-Faz̤l Bayhaqī (d. 470/1077) on historiography:
Historical information about the past is said to be of two kinds, with no third way about it: either one must hear them from someone or read about in a book. The necessary condition for the former is that the informant should be trustworthy and veracious, that one’s intellect should find it sound and authentic, and that it should be confirmed by the Word of God “Don’t give credence to any reports that are not acceptable to your judgement”. The same goes for a book: for whatever one reads in a book, so long as it is not rejected as implausible by one’s intellect, is held as being true by the reader, and the wise will also listen to it and take it in. (Bosworth's translation, vol. 2, pp. 370-71).