Time for a refresher course: How to avoid making mistakes in understanding events of the past

From Ibn Khaldun in "An Arab Philosophy of History" translated and arranged by Charles Issawi. Though this was written sometime in the the late 1370s, it is as relevant today as it must have been then.

All records, by their very nature, are liable to error....

The first of these is partisanship towards a creed or opinion. ....should the mind...be biassed in favour of an opinion or creed, it at once accepts every favourable piece of information concerning this opinion. Therefore, partisanship acts as a blinker to the mind, preventing it from investigating and criticizing and inclining it to the reception and transmission of error.

The second factor conducive to error is over-confidence in one's
sources. Such sources should be accepted only after thorough investigation involving the criticism of falsehoods and the correction of distortion.

A third factor is the failure to understand what is intended. Thus many a chronicler falls into error by failing to grasp the real meaning of what he has seen or heard and by relating the event according to what he thinks or imagines.

A fourth source of error is a mistaken belief in the truth. This happens often, generally taking the form of excessive faith in the authority of one's sources.

A fifth factor is the inability rightly to place an event in its real
context, owing to the obscurity and complexity of the situation. The chronicler contents himself with reporting the event as he saw it, thus distorting its significance.

A sixth factor is the very common desire to gain the favour of those of high rank, by praising them, by spreading their fame, by flattering them, by embellishing their doings and by interpreting in the most favourable way all their actions.

The seventh cause of error, and the most important of all, is the
ignorance of the laws governing the transformations of human society. For every single thing, whether it be an object or an action, is subject to a law governing its nature and any changes that may take place in it. If, therefore, the historian understands the nature of events and of changes that occur in the world, and the conditions governing them, such knowledge will help him more than anything else to clarify any record and to distinguish the truths it contains from the falsehoods. . . .

. . . Another cause of error is exaggeration. . . . Thus we find that most of our contemporaries give free rein to their imagination, follow the whisperings of exaggeration, and transgress the limitations of customary experience, when speaking of the armies of contemporary states, or of states which existed in the recent past; or when discussing the troops of Muslim or Christian nations; or when enumerating the revenues of kings, or the taxes or dues levied by them; or when estimating the expenditure of the wealthy, or the fortunes of the rich....

The real cause of this error is that men's minds are fond of all that is strange and unusual, and that the tongue easily slips into exaggeration, while the investigator and critic is apt to overlook things, so that he does not try to check his statements or weigh them up in a fair and critical spirit of enquiry and investigation, but rather gives his imagination a free rein and lets his tongue loose in a pasture of falsehoods. . . .