(2) Before drawing any conclusions it remains to make sure whether the _different_ observations of the same fact are entirely _independent_; for it is possible that one may have influenced another to such a degree that their agreement is inconclusive. We have to guard against the following cases:-- (_a_) The different observations have been made by the same author, who has recorded them either in the same or in different documents; special reasons must then be had before it can be assumed that the author really made the observation afresh, and did not content himself with merely repeating a single observation. (_b_) There were several observers, but they commissioned one of their number to write a single document. We have to ascertain whether the document merely gives the statements of the writer, or whether the other observers checked his work. (_c_) Several observers recorded their observations in different documents, but under similar conditions. We must apply the list of critical questions in order to ascertain whether they were not all subject to the same influences, predisposing to falsehood or error; whether, for example, they had a common interest, a common vanity, or common prejudices. The only observations which are certainly independent are those which are contained in different documents, written by different authors, who belonged to different groups, and worked under different conditions. Cases of perfectly conclusive agreement are thus rare, except in reference to modern periods. The possibility of proving an historical fact depends on the number of independent documents relating to it which have been preserved, and the preservation of the documents is a matter of chance; this explains the share which chance has in the formation of historical science. The facts which it is possible to establish are chiefly those which cover a large extent of space or time (sometimes called _general_ facts), customs, doctrines, institutions, great events; they were easier to observe than the others, and are now easier to prove. Historical method is not, however, essentially powerless to establish facts of short duration and limited extent (those which are called _particular facts_), such as a saying, a momentary act. It is enough that several persons should have been present when the fact occurred, that they should have recorded it, and that their writings should have come down to us. We know what were the words which Luther uttered at the Diet of Worms; we know that he did not say what tradition puts in his mouth. This concurrence of favourable conditions becomes more and more frequent with the organisation of newspapers, of shorthand writers, and of depositories of documents.