This concentration is performed without effort in the case of habits which have to do with forms (language, handwriting), and in the case of all intellectual habits; those who practised these habits have already given them expression in formulæ, which we have only to collect. The same holds of these institutions which are sanctioned by expressly formulated rules (regulations, laws, private statutes). Accordingly the special branches of history were the first to yield methodical formulæ. On the other hand, these special branches do not go beyond superficial and conventional facts, they do not reach the real actions and thoughts of men: in language they deal with written words, not the real pronunciation; in religion with official dogmas and rites, not with the real beliefs of the mass of the people; in morals with avowed precepts, not with the effective ideals; in institutions with official rules, not with the real practice. On all these subjects the knowledge of conventional forms must some day be supplemented by a parallel study of the real habits. It is much more difficult to embrace in a single formula a habit which is composed of real actions, as is the case with economic phenomena, private life, politics; for we have to find in the different actions those common characteristics which constitute the habit; or, if this work has already been done in the documents, and condensed into a formula (the most common case), we must criticise this formula in order to make sure that it really represents a homogeneous habit. The same difficulty occurs in constructing the formula for a group; we have to describe the characteristic common to all the members of the group and to find a collective name which shall exactly designate it. In documents there is no lack of names of groups; but, as they have their origin in usage, many of them correspond but ill to the real groups; we have to criticise these names to fix their precise meaning, sometimes to correct their application. This first operation should yield formulæ expressive of the conventional and real characteristics of all the habits of the different groups. In order to fix the precise _extent_ of a habit we shall seek the most distant points where it appears (this will give the area of distribution), and the region where it is most common (the centre). Sometimes the operation takes the form of a map (for example the map of the _tumuli_ and the _dolmens_ of France). It will also be necessary to indicate the groups of men who practised each habit, and the sub-groups in which it was most pronounced. The formula should also indicate the _duration_ of the habit. We shall look for the extreme cases, the first and the last appearance of the form, the doctrine, the usage, the institution, the group. But it will not be enough to note the two isolated cases, the earliest and the most recent; we must ascertain the period in which it was really active.