Social, cultural, and political conditions change over time, and secondary references may view past events with a modern eye. This is good, because the writer can evaluate an event or period as a whole, taking all the elements into consideration with the benefit of hindsight. However, in order to fully understand and appreciate the people and the events of a particular period of time, primary references are necessary. Objectivity When using primary references, be aware that not all of them will be wholly objective. Many of them might be opinionated or biased, putting forth one side of a story instead of balancing both sides.
Some references can be pure propaganda.
Again taking the social or cultural conditions into consideration, you will find that people's ways of thinking today can be quite different from the period in which you are researching. An article about women that was written in the late 1800s, for example, may put forward the opinion that women should not be allowed to work because it could undermine family values, or that women should not have the right to vote because they aren't capable of making such intelligent decisions. In the late 1800s, many people felt that way, but that doesn't mean they were right. Opinions have changed, and we now know better. Other sources may be biased because the writer is deliberately putting forth a particular opinion or version of events for their own reasons.