The quantity of documents in existence, if not of known documents, is given; time, in spite of all the precautions which are taken nowadays, is continually diminishing it; it will never increase. History has at its disposal a limited stock of documents; this very circumstance limits the possible progress of historical science. When all the documents are known, and have gone through the operations which fit them for use, the work of critical scholarship will be finished. In the case of some ancient periods, for which documents are rare, we can now see that in a generation or two it will be time to stop. Historians will then be obliged to take refuge more and more in modern periods. Thus history will not fulfil the dream which, in the nineteenth century, inspired the romantic school with so much enthusiasm for the study of history: it will not penetrate the mystery of the origin of societies; and, for want of documents, the beginnings of the evolution of humanity will always remain obscure. The historian does not collect by his own observation the materials necessary for history as is done in the other sciences: he works on facts the knowledge of which has been transmitted by former observers. In history knowledge is not obtained, as in the other sciences, by direct methods, it is indirect. History is not, as has been said, a science of observation, but a science of reasoning. In order to use facts which have been observed under unknown conditions, it is necessary to apply criticism to them, and criticism consists in a series of reasonings by analogy. The facts as furnished by criticism are isolated and scattered; in order to organise them into a structure it is necessary to imagine and group them in accordance with their resemblances to facts of the present day, an operation which also depends on the use of analogies. This necessity compels history to use an exceptional method. In order to frame its arguments from analogy, it must always combine the knowledge of the particular conditions under which the facts of the past occurred with an understanding of the general conditions under which the facts of humanity occur. Its method is to draw up special _tables_ of the facts of an epoch in the past, and to apply to them sets of _questions_ founded on the study of the present.