The interpretation of sources and historical facts is subjective, but that does not mean that all interpretation are of the same quality and are thus equally valid. Every bit of historical data that we have came into existence in a specific context, often created by a person who had their own way of looking at the world. Each source also, to a greater or lesser degree, portrays the actions of people who acted on their own initiative for their own reasons.

Source analysis is not just applying your own ideas to what you see in a way that supports your argument. In the “Opinion” blog post I said that intelligent people are full of informed opinions, which is important for understanding how we arrive at arguments. In source analysis, supposedly informed opinions are of little use if you don’t have the right contextual knowledge. There is an objective past that we glimpse in pieces, and actions are easily misunderstood without having general knowledge of that past. A lot of poor source analysis stems from poor quality background research, where people too quickly come to a conclusion.
This is where working with someone who knows more than you comes in handy. One of the best things about a university education is that students’ work is read by experts in the field (or TAs suffering from imposter syndrome) who are skilled at giving feedback. If you want to develop research and writing skills for free you can be active on online forums and get plenty of feedback from random commenters, but there is little guarantee of receiving quality feedback.
If you willfully or ignorantly misinterpret source material it undermines the strength of your thesis, even if the argument is a strong one and you found some quality sources. However, information gleaned from sources often have their own basic premises that can conflict with the basic premises of your argument. Too many students think that certain sources inherently conflict with their argument and must therefore not be used. I disagree with that. Interpret the source fairly and to the best of your ability without misrepresenting it, then explain how it still supports your argument.
When you become skilled at analyzing sources and incorporating potentially conflicting views into your paper, then you broaden your research scope because you don’t have to exclude sources that might undermine your argument.
Source analysis is what turns the historical record into history. Because there are a limited number of ways to subjectively interpret the objective past through passive sources, there are only a limited number of possible histories that closely align with the past. Yet within those constraints, historians still manage to have compelling and never-ending debates with each other. Historians will never agree because historians don’t have to be in agreement for their ideas to be valid.