Why are some sources better than others? There are many ways to answer this question, but for the historian, it begins with an understanding of the research playing field. Aspiring historians learn to distinguish primary and secondary sources along a kind of baseball diamond.
This field illustrates a key insight. Primary sources are almost always most reliable the closer they are to the point of creation, but secondary sources tend to become more useful the further they are away from events. In both cases, it is a question of distance. Proximity correlates with truth in first-hand evidence, but the passage of time helps encourage broader perspective in the development of secondary sources.