One of the huge mistakes musicologists and historians make is not realizing how LITTLE has been preserved through the centuries. They need to realize that we currently possess mere fragments, many of them preserved by accident. Yet, historians and musicologists so often forget this fact. So often, they make the terrible mistake of looking at what has survived and making assumptions based only on these things. A more sensible approach is to realize and admit that we have very little, and what has been preserved does not necessarily represent the entirety of (for example) the Gregorian repertoire.
More at The Propers, Installment 6