Others, well understanding the advantages of systematic classification, have proposed to fit their materials, as fast as collected, into their appropriate places in a prearranged scheme. For this purpose they use note-books of which every page has first been provided with a heading. Thus all the entries of the same kind are close to one another. This system leaves something to be desired; for additions will not always fit without inconvenience into their proper place; and the scheme of classification, once adopted, is rigid, and can only be modified with difficulty. Many librarians used to draw up their catalogues on this plan, which is now universally condemned. There is a still more barbarous method, which need not receive more than passing mention. This is simply to register documents in the memory without taking written notes. This method has been used. Historians endowed with excellent memories, and lazy to boot, have indulged this whim, with the result that their quotations and references are mostly inexact. The human memory is a delicate piece of registering apparatus, but it is so little an instrument of precision that such presumption is inexcusable. Every one admits nowadays that it is advisable to collect materials on separate cards or slips of paper. The notes from each document are entered upon a loose leaf furnished with the precisest possible indications of origin. The advantages of this artifice are obvious: the detachability of the slips enables us to group them at will in a host of different combinations; if necessary, to change their places: it is easy to bring texts of the same kind together, and to incorporate additions, as they are acquired, in the interior of the groups to which they belong. As for documents which are interesting from several points of view, and which ought to appear in several groups, it is sufficient to enter them several times over on different slips; or they may be represented, as often as may be required, on reference-slips. Moreover, the method of slips is the only one mechanically possible for the purpose of forming, classifying, and utilising a collection of documents of any great extent. Statisticians, financiers, and men of letters who observe, have now discovered this as well as scholars.