Answer: Everyone is capable of doing and seeing only what it is possible for them to do or see and also it is human nature to be biased they compare the sources and try to glean them.
Explanation: Historical sources are often biased, not limited to the past. Historians have a point of view and they help to understand the past, they have particular agenda and they may want to prove their superiority. Bias in historical sources can be found in what is left out.