In history there is nothing like this. One is apt to say that history is the "vision" of past events, and that it proceeds by "analysis": these are two metaphors, dangerous if we suffer ourselves to be misled by them.[181] In history we see nothing real except paper with writing on it--and sometimes monuments or the products of art or industry. The historian has nothing before him which he can analyse physically, nothing which he can destroy and reconstruct. "Historical analysis" is no more real than is the vision of historical facts; it is an abstract process, a purely intellectual operation. The analysis of a document consists in a _mental_ search for the items of information it contains, with the object of criticising them one by one. The analysis of a fact consists in the process of distinguishing _mentally_ between its different details (the various episodes of an event, the characteristics of an institution), with the object of paying special attention to each detail in turn; that is what is called examining the different "aspects" of a fact,--another metaphor. The human mind is vague by nature, and spontaneously revives only vague collective impressions; to impart clearness to these it is necessary to ask what individual impressions go to form a given collective impression, in order that precision may be attained by a successive consideration of them. This is an indispensable operation but we must not exaggerate its scope. It is not an objective method which yields a knowledge of real objects; it is only a subjective method which aims at detecting those abstract elements which compose our impressions.[182] From the very nature of its materials history is necessarily a subjective science. It would be illegitimate to extend to this intellectual analysis of subjective impressions the rules which govern the real analysis of real objects. History, then, must guard against the temptation to imitate the method of the biological sciences. Historical facts are so different from the facts of the other sciences that their study requires a different method. III. Documents, the sole source of historical knowledge, give information on three categories of facts: (1) _Living beings and material objects._ Documents make us acquainted with the existence of human beings, physical conditions, products of art and industry. In all these cases physical facts have been brought before the author by physical perception. But we have before us nothing but intellectual phenomena, facts seen "through the author's imagination," or, to speak accurately, mental images representative of the author's impressions--images which we form on the _analogy_ of the images which were in his mind. The Temple at Jerusalem was a material object which men saw, but we cannot see it now; all we can now do is to form a mental image of it, analogous to that which existed in the minds of those who saw and described it.