Sources are made by people, and people have feelings, loyalties and reasons for what they record. This means a source can be biased: it may show only one side of a story. Historians must therefore judge how far to trust each source before using it.
What bias means
Bias is not the same as lying. A source can be completely honest and yet still one-sided, because the person only saw part of the events or wanted to present things in a certain way. Two eyewitnesses to the very same event can describe it quite differently, and both can be sincere. So a good historian does not simply throw out a biased source; instead they ask what it is still useful for.
Testing reliability
To judge how far to trust a source, historians ask questions like these:
- Who made the source, and were they actually present?
- Did they have a reason to exaggerate or to leave things out?
- Does other evidence agree or disagree with it?
- How long after the event was it made?
A source that fails these tests is not useless. A boastful royal account, for example, can still be reliable evidence of how a ruler wished to be seen by others.
Why interpretations differ
An interpretation is a historian's explanation of the past. Historians can study the same events yet reach different conclusions because they ask different questions, have access to different sources, or give more weight to some evidence than others. Different interpretations are a normal and healthy part of history, not a sign that someone must be wrong.
Being fair
Good historians try to understand people in the past on their own terms, and to present more than one point of view before judging. Weighing evidence fairly, without deciding the answer in advance, is the true mark of careful history.
Ask not only "Is this true?" but "Whose view is this, and what have they left out?"