Historical method is the collection of techniques and guidelines that historians use to research and write histories of the past. Secondary sources, primary sources and material evidence such as that derived from archaeology may all be drawn on, and the historian's skill lies in identifying these sources, evaluating their relative authority, and combining their testimony appropriately in order to construct an accurate and reliable picture of past events and environments. In the philosophy of history, the question of the nature, and the possibility, of a sound historical method is raised within the sub-field of epistemology. The study of historical method and of different ways of writing history is known as historiography. Source criticism Source criticism (or information evaluation) is the process of evaluating the qualities of an information source, such as its validity, reliability, and relevance to the subject under investigation. Gilbert J. Garraghan and Jean Delanglez divide source criticism into six inquiries: When was the source, written or unwritten, produced (date)? Where was it produced (localization)? By whom was it produced (authorship)? From what pre-existing material was it produced (analysis)? In what original form was it produced (integrity)? What is the evidential value of its contents (credibility)? The first four are known as higher criticism; the fifth, lower criticism; and, together, external criticism. The sixth and final inquiry about a source is called internal criticism. Together, this inquiry is known as source criticism. R. J. Shafer on external criticism: "It sometimes is said that its function is negative, merely saving us from using false evidence; whereas internal criticism has the positive function of telling us how to use authenticated evidence." Noting that few documents are accepted as completely reliable, Louis Gottschalk sets down the general rule, "for each particular of a document the process of establishing credibility should be separately undertaken regardless of the general credibility of the author." An author's trustworthiness in the main may establish a background probability for the consideration of each statement, but each piece of evidence extracted must be weighed individually. Procedures for contradictory sources Bernheim (1889) and Langlois & Seignobos (1898) proposed a seven-step procedure for source criticism in history: If the sources all agree about an event, historians can consider the event proven. However, majority does not rule; even if most sources relate events in one way, that version will not prevail unless it passes the test of critical textual analysis. The