Such attacks signal readers that he may have an interest in not portraying the past accurately, and hence may undermine his credibility, regardless of his reliability. An author who seems quite credible may be utterly unreliable. The author who takes a measured, reasoned tone and anticipates counter-arguments may seem to be very credible, when in fact he presents us with complete balderdash. Similarly, a reliable author may not always seem credible. It should also be clear that individual texts themselves may have portions that are more reliable and credible than others.
Neutrality refers to the stake an author has in a text.
In the example of the soldier who committed wartime atrocities, the author seems to have had a considerable stake in his memoir, which was the expunge his own guilt. In an utterly neutral document, the creator is not aware that she or he has any special stake in the construction and content of the document. Very few texts are ever completely neutral. People generally do not go to the trouble to record their thoughts unless they have a purpose or design which renders them invested in the process of creating the text. Some historical texts, such as birth records, may appear to be more neutral than others, because their creators seem to have had less of a stake in creating them.