Primary sources do not always yield up their secrets easily. One difficulty which occurs, especially with older sources, is that they can be incomplete, damaged or written in languages long since extinct. Such problems were experienced with both the Sumerian clay tablets and the Egyptian mathematical papyri. The confidence that we can place in such sources must, to a large extent be governed by the faith that we have in their translation and evaluation. Another point worth mentioning is that a primary source, even when correctly interpreted, may not be representative of a whole culture, and so we should be cautious when dealing with isolated finds.
Secondary sources are naturally suspect, often being the views of a single scholar.
These views well may be biased or derived from incomplete or incorrect evidence. Sometimes, authors are forced to change their views in the light of recent research, or because they honestly admit that they have been wrong. Some writers make it clear when they are presenting their own views, while others put them forward as well established 'facts'. Popular consensus changes as new discoveries come to light. Sometimes, only one or two original works are available on a particular topic, for instance Chinese mathematics, and so authors of general textbooks are forced either to accept these authorities, or else ignore the topic altogether.