The alphabetical arrangement is very convenient when the chronological and geographical arrangements are unsuitable. There are documents, such as the sermons, the hymns, and the secular songs of the middle ages, which are not precisely dated or localised. They are arranged in the alphabetical order of their _incipit_--that is, the words with which they begin.[100] The systematic order, or arrangement by subjects, is not to be recommended for the compilation of a _Corpus_ or of _regesta_. It is always arbitrary, and leads to inevitable repetition and confusion. Besides, given collections arranged in chronological, geographical, or alphabetical order, nothing more than the addition of a good table of contents is needed to make them available for all the purposes which would be served by a systematic arrangement. One of the chief rules of the art of _Corpus_ and _regesta_-making, that great art which has been carried to such perfection in the second half of the nineteenth century,[101] is to provide these collections, whatever the grouping adopted, with a variety of tables and indexes of a kind to facilitate the use of them: _incipit_ tables in chronological _regesta_ which lend themselves to such treatment, indexes of names and dates in _regesta_ arranged by order of _incipit_, and so on. _Corpus_ and _regesta_-makers collect and classify for the use of others documents in which, at any rate in _all_ of which, they have no direct interest, and are absorbed in this labour. Ordinary workers, on the other hand, only collect and classify materials useful for their individual studies. Hence certain differences arise. For example, the arrangement by subjects, on a predetermined system, which is so little to be recommended for great collections, often provides those who are composing monographs on their own account with a scheme of classification preferable to any other. But it will always be well to cultivate the mechanical habits of which professional compilers have learnt the value by experience: to write at the head of every slip its date, if there is occasion for it, and a heading[102] in any case; to multiply cross-references and indices; to keep a record, on a separate set of slips, of all the sources utilised, in order to avoid the danger of having to work a second time through materials already dealt with. The regular observance of these maxims goes a great way towards making scientific historical work easier and more solid. The possession of a well-arranged, though incomplete, collection of slips has enabled M. B. Hauréau to exhibit to the end of his life an undeniable mastery over the very special class of historical problems which he studied.[103] CHAPTER V CRITICAL SCHOLARSHIP AND SCHOLARS The sum of the operations described in the preceding chapters (restoration of texts, investigation of authorship, collection and classification of verified documents) constitutes the vast domain of external criticism, or critical scholarship.