As time passes, societies change, and one of the most significant changes that societies endure is that of changing its values. Some principles or actions that societies valued in the past might not be thought of in the same way in the present or in the future.
For example, behaviours such as slavery or imperialism were seen in a positive light by many people in the past, whereas they are now almost universally condemned.
As perceptions change, history is affected too. Historians are not immune to such changes in perception, and their own biases might affect their interpretation of history. Moreover, common people might also understand history in a way that is heavily influenced by their own personal biases.
