You might want to take a look at my friend Mark Safranski’s post on the limits and constraints of historiography:
Historians are prisoners of their primary sources. Without them they are theorists or mere speculators. Too few, as with ancient history, and the historian is engaged in the same sort of guesswork as archaelogists and paleontologists or they are reduced to ideological theorizing, something historians are supposed to hold as suspect and as fit only for political scientists.
Too many sources, as with any modern presidential administration, and the sheer number makes it difficult to find critical evidence or draw upon a defensibly representative sample. Ronald Reagan supposedly signed one million documents in his eight years in the White House. True or not, the figure would represent a fraction of what his administration generated. Many documents of great importance are nonetheless not important enough at the time to cross the desk of the President of the United States.
The need to craft a narrative, imposes other restraints. There is no “history” unless the results of a historian’s work are disseminated and understood, challenged and defended.
Don’t miss the video to which Mark links, a satirical piece set 1,000 years in the future on future historians’ views of the Beatles. It encapsulates my views of ancient history nearly perfectly.