The right method of conducting the operation is as follows: (1) We must fix the precise limits of the field within which we intend to generalise (that is, to assume the similarity of all the cases), we must determine the country, the group, the class, the period as to which we are to generalise. Care must be taken not to make the field too large by confusing a part with the whole (a Greek or Germanic people with the whole Greek or Germanic race). (2) We must make sure that the facts lying within the field resemble each other in the points on which we wish to generalise, and therefore we have to distrust those vague names under which are comprehended groups of very different character (Christians, French, Aryans, Romans). (3) We must make sure that the facts from which we propose to generalise are representative samples, that they really belong to the field of investigation, for it does happen sometimes that men or facts are taken as specimens of one group when they really belong to another. Nor must they be exceptional, as is to be presumed in all cases when the conditions are exceptional; authors of documents tend to record by preference those facts which surprise them, hence exceptional cases occupy in documents a space which is out of proportion to their real number; this is one of the chief sources of error. (4) The number of samples necessary to support a generalisation is the greater the less ground there is for supposing a resemblance between all the cases occurring within the field of investigation. A small number may suffice in treating of points in which men tend to bear a strong resemblance to each other, either by imitation and convention (language, rites, ceremonies), or from the influence of custom and obligatory regulations (social institutions, political institutions in countries where the authorities are obeyed). A large number is requisite for facts where individual initiative plays a more important part (art, science, morality), and sometimes, as in respect of private conduct, all generalisation is as a rule impossible. VI. Descriptive formulæ are in no science the final result of the work. It still remains to group the facts in such a way as to bring out their collective import, it still remains to search for their mutual relations; these are the general conclusions. History, by reason of the imperfection of its mode of acquiring knowledge, needs, in addition, a preliminary operation for determining the bearing of the knowledge acquired.[207]