The correct answer to this open question is the following.
When analyzing a source, historians want to remain neutral. In other words, they don’t want to be biased by their personal feelings and opinions. They know that different sources may reveal different conclusions.
For the success of the research, historians need to contemplate al kind of primary and secondary sources that are valid. They cannot biased information due to personal beliefs, religion, background or previous experiences. They have to remain neutral and be open to new ideas, discoveries or points of view. That is how history can be best understood. If they biased or use the same information, they can't reach new conclusions.