to
be true; a statement whose truth is probable may, after all, be false.
Instances occur continually, and we are never sufficiently well
acquainted with the conditions under which the observation was made to
_know_ whether it was made ill or well.

In order to obtain a definitive result we require a final operation.
After passing through the ordeal of criticism, statements present
themselves as probable or improbable. But even the most probable of
them, taken by themselves, remain mere probabilities: to pass from them
to categorical propositions in scientific form is a step we have no
right to take; a proposition in a science is an assertion not open to
debate, and that is what the statements we have before us are not. It is
a principle common to all sciences of observation not to base a
scientific conclusion on a single observation; the fact must have been
corroborated by several independent observations before it is affirmed
categorically. History, with its imperfect modes of acquiring
information, has less right than any other science to claim exemption
from this principle. An historical statement is, in the most favourable
case, but an indifferently made observation, and needs other
observations to corroborate it.

It is by combining observations that every science is built up: a
scientific fact is a centre on which several different observations
converge.[170] Each observation is subject to chances of error which
cannot be entirely eliminated; but if several observations agree, this
can hardly be in virtue of a common error: the more probable explanation
of the agreement is that the observers have all seen the same reality
and have all described it correctly. Errors are personal and tend to
diverge; it is the correct observations that agree.

Applied to history, this principle leads to a last series of operations,
intermediate between purely analytical criticism and the synthetic
operations--the comparison of statements.

We begin by classifying the results yielded by critical analysis in such
a way as to bring together those statements which relate to the same
fact. The operation is facilitated mechanically by the method of slips.
Either each statement has been entered on a separate slip, or else a
single slip has been assigned for each fact, and the different
statements relating to it entered upon the slip as met with in the
course of reading. By bringing the statements together we learn the
extent of our