Gibbon offers an explanation for why the Roman Empire fell, a task made difficult by a lack of comprehensive written sources, though he was not the only historian to tackle the subject. Most of his ideas are directly taken from what few relevant records were available: those of the Roman moralists of the 4th and 5th centuries.

Art must be the faithful expression of a society, since it represents it by its works as it has created them -- undeniable witnesses of its spirit and manners for future generations. But it must be acknowledged that art is only the consequence of the ideas which it expresses; it is the fruit of civilization, not its origin. To understand the Middle Ages and the Renaissance, it is necessary to go back to the source of its art, and to know the life of our fathers; these are two inseparable things, which entwine one another, and become complete one by the other. - Preface.