ostulate of all the documentary sciences. If former humanity did not
resemble the humanity of to-day, documents would be unintelligible.
Starting from this assumed resemblance, the historian forms a mental
representation of the bygone facts of history similar to his own
recollection of the facts he has witnessed.

This operation, which is performed unconsciously, is one of the
principal sources of error in history. The things of the past which are
to be pictured in imagination were not wholly similar to the things of
the present which we have seen; we have never seen a man like Caesar or
Clovis, and we have never experienced the same mental states as they. In
the established sciences it is equally true that one man will work on
facts which another has observed, and 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 in language