We begin by classifying the results yielded by critical analysis in such a way as to bring together those statements which relate to the same fact. The operation is facilitated mechanically by the method of slips. Either each statement has been entered on a separate slip, or else a single slip has been assigned for each fact, and the different statements relating to it entered upon the slip as met with in the course of reading. By bringing the statements together we learn the extent of our information on the fact; the definitive conclusion depends on the relation between the statements. We have, then, to study separately the different cases which may occur. III. Most frequently, except in contemporary history, the documents only supply a single statement on a given fact. In such a case all the other sciences follow an invariable rule: an isolated observation is not admitted into science; it is quoted (with the observer's name), but no conclusions are drawn from it. Historians have no avowable motive for proceeding otherwise. When a fact is supported by no more than the statement of a single man, however honest he may be, historians ought not to assert it, but to do as men of science do--give the reference (Thucydides states, Cæsar says that ...); this is all they have a right to affirm. In reality they all retain the habit of stating facts, as was done in the middle ages, on the _authority_ of Thucydides or of Cæsar; many are simple enough to do so in express terms. Thus, allowing themselves to be guided by natural credulity, unchecked by science, historians end by admitting, on the insufficient presumption afforded by a unique document, any statement which does not happen to be contradicted by another document. Hence the absurd consequence that history is more positive, and seems better established in regard to those little known periods which are represented by a single writer than in regard to facts known from thousands of documents which contradict each other. The wars of the Medes known to Herodotus alone, the adventures of Fredegonda related by none but Gregory of Tours, are less subject to discussion than the events of the French Revolution, which have been described by hundreds of contemporaries. This is a discreditable state of things which cannot be ended except by a revolution in the minds of historians. IV. When we have several statements relating to the same fact, they may contradict each other or they may agree. In order to be certain that they really do contradict each other, we have to make sure that they do actually relate to the same fact. Two apparently contradictory statements may be merely parallel; they may not relate exactly to the same moment, the same place, the same persons, the same episodes of an event, and they may be both correct.[171] We must not, however, infer that they confirm each other; each comes under the category of unique statements.