Evaluating sources

Historians use these criteria to decide what weight to place on a piece of evidence:

• Is it original? Documents, images and maps can be copied, so one needs to identify the original. Errors could have crept in during copying or translation. Or a copyist may have made alterations and additions intended to be helpful, but which are anachronistic.
• How close is the author in time and place to what he is describing/depicting? Eye-witness accounts are best. The author may not tell us where and when he is writing, let alone what he actually witnessed, so this may have to be deduced from clues in the text.
• Was he or she a good observer? Intelligent, observant and expert testimony is to be preferred. Again the clues to this may be embedded in the text/image. In general the more detail, the more we feel that this observer has paid close attention, but this is not an absolute rule. Folktales tend to get embroidered as they go along. The type of detail and source therefore need to be taken into account. On a bird's-eye view, do all the houses look the same, or is each depicted differently? The latter is more convincing.
• Was he accurate? Anyone can make a mistake. Apparently reliable, detailed, expert testimony, for example the porch has V Corinthian columns may incorporate a careless error - four or six being intended. Common sense and cross-checking with other sources help to shake out errors. In this case we know that such a structure would normally have an even number of columns, so we smell an error right away.
• How objective was he? Possible biases include political, religious and personal. For example a monastic chronicler is liable to lambaste as a hell-bent monster of depravity any person who infringed the privileges of the church.
• What was the purpose of the document, map, image, object? A landscape by Turner was intended for visual effect and he might rearrange the scenery to that end, unless he was being paid to record what he saw. Since builders wanted paying, a building account should include every item of work, while sale particulars may give a glowing account that glosses over that little problem with the sewage.
• Is it part of a series? If so, does it include standard linguistic formulae or visual conventions? For example, if a map-maker invariably uses a little square with a spire on top to indicate the presence of a church, it would be unwise to assume that any particular church on the map had a spire at that date.