Events can be empirically known in two ways only: by direct observation while they are in progress; and indirectly, by the study of the traces which they leave behind them. Take an earthquake, for example. I have a direct knowledge of it if I am present when the phenomenon occurs; an indirect knowledge if, without having been thus present, I observe its physical effects (crevices, ruins), or if, after these effects have disappeared, I read a description written by some one who has himself witnessed the phenomenon or its effects. Now, the peculiarity of "historical facts"[55] is this, that they are only known indirectly by the help of their traces. Historical knowledge is essentially indirect knowledge. The methods of historical science ought, therefore, to be radically different from those of the direct sciences; that is to say, of all the other sciences, except geology, which are founded on direct observation. Historical science, whatever may be said,[56] is not a science of observation at all. The facts of the past are only known to us by the traces of them which have been preserved. These traces, it is true, are directly observed by the historian, but, after that, he has nothing more to observe; what remains is the work of reasoning, in which he endeavours to infer, with the greatest possible exactness, the facts from the traces. The document is his starting-point, the fact his goal.[57] Between this starting-point and this goal he has to pass through a complicated series of inferences, closely interwoven with each other, in which there are innumerable chances of error; while the least error, whether committed at the beginning, middle, or end of the work, may vitiate all his conclusions. The "historical," or indirect, method is thus obviously inferior to the method of direct observation; but historians have no choice: it is the _only_ method of arriving at past facts, and we shall see later on[58] how, in spite of these disadvantages, it is possible for this method to lead to scientific knowledge. The detailed analysis of the reasonings which lead from the inspection of documents to the knowledge of facts is one of the chief parts of Historical Methodology. It is the domain of criticism. The seven following chapters will be devoted to it. We shall endeavour, first of all, to give a very summary sketch of the general lines and main divisions of the subject.