I. We may distinguish two species of documents. Sometimes the past event has left a material trace (a monument, a fabricated article). Sometimes, and more commonly, the trace is of the psychological order--a written description or narrative. The first case is much simpler than the second. For there is a fixed relation between certain physical appearances and the causes which produced them; and this relation, governed by physical laws, is known to us.[59] But a psychological trace, on the other hand, is purely symbolic: it is not the fact itself; it is not even the immediate impression made by the fact upon the witness's mind, but only a conventional symbol of that impression. Written documents, then, are not, as material documents are, valuable by themselves; they are only valuable as signs of psychological operations, which are often complicated and hard to unravel. The immense majority of the documents which furnish the historian with starting-points for his reasonings are nothing else than traces of psychological operations. This granted, in order to conclude from a written document to the fact which was its remote cause--that is, in order to ascertain the relation which connects the document with the fact--it is necessary to reproduce the whole series of intermediate causes which have given rise to the document. It is necessary to revive in imagination the whole of that series of acts performed by the author of the document which begins with the fact observed by him and ends with the manuscript (or printed volume), in order to arrive at the original event. Such is the aim and such the process of critical analysis.[60] First of all we observe the document. Is it now in the same state as when it was produced? Has it deteriorated since? We endeavour to find out how it was made in order to restore it, if need be, to its original form, and to ascertain its origin. The first group of preliminary investigations, bearing upon the writing, the language, the form, the source, constitutes the special domain of EXTERNAL CRITICISM, or critical scholarship. Next comes INTERNAL CRITICISM: it endeavours, by the help of analogies mostly borrowed from general psychology, to reproduce the mental states through which the author of the document passed. Knowing what the author of the document has said, we ask (1) What did he mean? (2) Did he believe what he said? (3) Was he justified in believing whatever he did believe? This last step brings the document to a point where it resembles the data of the objective sciences: it becomes an observation; it only remains to treat it by the methods of the objective sciences. Every document is valuable precisely to the extent to which, by the study of its origin, it has been reduced to a well-made observation. II. Two conclusions may be drawn from what we have just said: the extreme complexity and the absolute necessity of Historical Criticism.