en they belong to a different species,
and the special historian is obliged to borrow them from general
history.

In general history the periods should be divided according to the
evolution of several species of phenomena; we look for events which mark
an epoch simultaneously in several branches (the Invasion of the
Barbarians, the Reformation, the French Revolution). We may thus
construct periods which are common to several branches of evolution,
whose beginning and whose end are each marked by a single event. It is
thus that the traditional division of universal history into periods has
been effected. The sub-periods are obtained by the same process, by
taking for limits events which have produced consequences of secondary
importance.

The periods which are thus constructed according to the events are of
unequal duration. We must not be troubled by this want of symmetry; a
period ought not to be a fixed number of years, but the time occupied by
a distinct phase of evolution. Now, evolution is not a regular movement;
sometimes a long series of years passes without notable change, then
come moments of rapid transformation. On this difference Saint-Simon has
founded a distinction between _organic_ periods (of slow change) and
_critical_ periods (of rapid change).

CHAPTER III

CONSTRUCTIVE REASONING

I. The historical facts supplied by documents are never enough to fill
all the blanks in such schemes of classification and arrangement as we
have been considering. There are many questions to which no direct
answer is given by the documents; many features are lacking without
which the complete picture of the various states of society, of
evolutions and events, cannot be given. We are irresistibly impelled to
endeavour to fill up these gaps.

In the sciences of direct observation, when a fact is missing from a
series, it is sought for by a new observation. In history, where we have
not this resource, we seek to extend our knowledge by the help of
reasoning. Starting from facts known to us from the documents, we
endeavour to reach new facts by inference. If the reasoning be correct,
this method of acquiring knowledge is legitimate.

But experience shows that of all the methods of acquiring historical
knowledge, reasoning is the most difficult to employ correctly, and the
one which has introduced the most serious errors. It should not be used
without the safeguard of a number of precautions calculated to keep