uired.

(1) We begin by inquiring whether the statements are independent, or are
reproductions of one and the same observation. This inquiry is partly
the work of that part of external criticism which deals with the
investigation of sources;[173] but that investigation only touches the
relations between written documents, and stops short when it has
determined which passages of an author are borrowed from other authors.
Borrowed passages are to be rejected without discussion. But the same
work remains to be done in reference to statements which were not
committed to writing. We have to compare the statements which relate to
the same fact, in order to find out whether they proceeded originally
from different observers, or at least from different observations.

The principle is analogous to that employed in the investigation of
sources. The details of a social fact are so manifold, and there are so
many different ways of looking at the same fact, that two independent
observers cannot possibly give completely coincident accounts; if two
statements present the same details in the same order, they must be
derived from a common observation; different observations are bound to
diverge somewhere. We may often apply an _a priori_ principle: if the
fact was of such a nature that it could only be observed or reported by
a single observer, then all the accounts of it must be derived from a
single observation. These principles[174] enable us to recognise many
cases of different observations, and still more numerous cases of
observations being reproduced.

There remains a great number of doubtful cases. The natural tendency is
to treat them as if they were cases of independent observation. But the
scientific procedure would be the exact reverse of this: as long as the
statements are not proved to be independent we have no right to assume
that their agreement is conclusive.

It is only after we have determined the relations between the different
statements that we can begin to count them and examine into their
agreement. Here again we have to distrust the first impulse; the kind of
agreement which is really conclusive is not, as one would naturally
imagine, a perfect similarity between two narratives, but an occasional
coincidence between two narratives which only partially resemble each
other. The natural tendency is to think that the closer the agreement
is, the greater is its demonstrative power; we ought, on the contrary,
to a