The Age of the Encyclopedia began in 1751, with the first volume of the first edition of Denis Diderot's Encyclopedie, and ended in 1911, with the 11th edition of the Encyclopedia Britannica, shortly before the world went mad. An encyclopedia is a compilation of the world's knowledge; an encyclopedic age requires the twin beliefs that such a compilation is possible, and that it is worthwhile. By 1911 it was no longer quite possible, in truth, but the dream lived on. The advertisements for the 11th declared that it was "Everything Explained That Is Explainable." In any case, the firm conviction of the worth of such a project allayed any fears about its possibility.

Underlying these two beliefs, though not following them in strict logic, is the idea of progress, indeed of continuous progress. Compilation implies rational understanding, the purpose of which is to make further understanding, if not inevitable, at least possible. With further understanding we may expect further improvement of the human condition. The encyclopedic outlook is fundamentally Whig. However else Diderot's philosophes and the editors of the 11th might have differed, on this they were agreed. As J.B. Bury, a contributor, pointed out in The Idea of Progress, the very concept of continuous historical progress did not exist before the 18th century. After 1911, it could no longer be believed in.