As an historian, one of the first lessons you learn is that history is subjective. We base ourselves on (mostly) written sources, that are in themselves imperfect. It's all about interpretation.
You must consider your source. An eyewitness to any event will be looking at it from a certain perspective. Their account is going to be coloured by their own personal beliefs, their own place in what is happening. They might even be trying to deliberately get a certain point across. The historian must be aware of this and take it into account when analysing the data available. Sometimes two different sources can state facts that directly contradict each other. It is difficult to extract the truth.
Historians, though we supposedly strive for objectivity, are really no better anyway. History is not an exact science, and we cannot claim to have absolute facts. All we can try to do is interpret the information that we have, but we too, are going to be influenced by our own experiences and even just the time we live in. Language can be a big barrier, with words and phrasings changing meaning. Morals and values change over time, and things that were normal once have become despicable to us. For example, it is very difficult to carry out an academic discussion of slavery without getting into very emotional and thus subjective territory. One of my lecturers in my undergraduate degree regularly received death threats as a result of his work in that area.
History is a discussion without end.