which he must therefore represent to himself by analogy; but these facts are defined by precise terms which indicate what invariable elements ought to appear in the image. Even in physiology the notions which occur are sufficiently clear and fixed for the same word to evoke in the minds of all naturalists similar images of an organ or a movement. The reason is that each notion which has a name has been formed by a method of observation and abstraction in the course of which all the characteristics which belong to the notion have been precisely determined and described.
But in proportion as a body of knowledge is more nearly concerned with the invisible facts of the mind, its notions become more confused and its language less precise. Even the most ordinary facts of human life, social conditions, actions, motives, feelings, can only be expressed by vague terms (king, warrior, to fight, to elect). In the case of more complex phenomena, language is so indefinite that there is no agreement even as to the essential elements of the phenomena. What are we to understand by a tribe, an army, an industry, a market, a revolution? Here history shares the vagueness common to all the sciences of humanity, psychological or social. But its indirect method of representation by mental images renders this vagueness still more dangerous. The historical images in our minds ought, then, to reproduce at least the essential features of the images which were in the minds of the direct observers of past facts; but the terms in which they expressed their mental images never tell us exactly what these essential elements were.
Facts which we did not see, described