between independent observations. Two operations are thus required.
(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; 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 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