Five thousand years ago there may be many gaps in the historical record, fifty years ago there may be so much information that it is virtually impossible for a single historian to digest the material.
Physical evidence from the past may provide firm evidence for what "really happened". Thus an archaeological dig of a battlefield may reveal a wealth of information about the type ammunition used, the number of dead and even the type of food that the armies were eating. However, even with a wealth of physical evidence, the historian's task is to place it in context. Thus, what does the type of food that was eaten by an army mean to the overall reality of what was occurring at a time
Another difficulty with discovering "what really happened" is that the historian needs to decide from whose viewpoint are the occurrences being seen. The traditional, "great man" view of history, which tells of the happenings that occurred to Kings, Queens, Emperors, Presidents, Prime Ministers . . . . that is very different from a history of the ordinary or poor people. The latter may be virtually unaware of what is happening at the national level, while the former may ignore the plight of most of the people that they rule. So "what really happened" depends upon the point of view being taken. "Happening" is a multiple occurrence and has multiple dimensions according to the different groups and individuals being considered.
In more recent history, the historian faces the challenge of having perhaps too much information. For example, a historian studying the assassination of President John F Kennedy will find hundreds of thousands of documents at his disposal. Indeed, a historian could go through a lifetime reading all the documentation and never come to an end. Thus, rather than trying to piece together the past from scattered and incomplete knowledge, the historian needs to select from among that knowledge. A degree of selection may also lead to a degree of bias as the historian is almost bound to choose those documents and sources that are of most interest to him or which support a preconceived notion regarding an event.
2. To what extent can a historian be objective
A historian should at least try to be as objective as possible, but absolute objectivity is impossible. Thus the first task is to not 'judge" the historical period or figure being considered by the standards of your own time. Considering a figure such as Henry VIII through the eyes of the ethics and standards of the Twenty-First Century is both futile and debilitating to the historical process. A King (or an ordinary man) should be considered within the context of his time in order to understand what, how and why things occurred.
In recent years the so called "new history" has often tried to re-interpret historical events according to the standards of today. Thus Feminist history seeks to explore the subjugation and oppression of women, gay history does the same for gay people etc. While a lot of fascinating scholarship appears within these genres of history, the actual sense of the place and time being considered often becomes lost within the ideological vigor of the historian. Such historians often seem to pride themselves on the fact that they are not being objective.
To be fair, those proponents of new history would suggest that traditional historians are just as un-objective through their uncritical analysis of patriarchal, sexist, homophobic societies. The new historians claim that not to