om Suetonius: they have nothing to do with the history of the ninth
century; how if the fact had not been discovered? An event is attested
three times, by three chroniclers; but these three attestations, which
agree so admirably, are really only one if it is ascertained that two of
the three chroniclers copied the third, or that the three parallel
accounts have been drawn from one and the same source. Pontifical
letters and Imperial charters of the middle ages contain eloquent
passages which must not be taken seriously; they are part of the
official style, and were copied word for word from chancery formularies.

It belongs to the investigation of authorship to discover, as far as
possible, the _sources_ utilised by the authors of documents.

The problem thus presented to us has some resemblance to that of the
restoration of texts of which we have already spoken. In both cases we
proceed on the assumption that identical readings have a common source:
a number of different scribes, in transcribing a text, will not make
exactly the same mistakes in exactly the same places; a number of
different writers, relating the same facts, will not have viewed them
from exactly the same standpoint, nor will they say the same things in
exactly the same language. The great complexity of historical events
makes it extremely improbable that two independent observers should
narrate them in the same manner. We endeavour to group the documents
into families in the same way as we make families of manuscripts.
Similarly, we are enabled in the result to draw up genealogical tables.
The examiners who correct the compositions of candidates for the
bachelor's degree sometimes notice that the papers of two candidates who
sat next each other bear a family likeness. If they have a mind to find
out which is derived from the other, they have no difficulty in doing
so, in spite of the petty artifices (slight modifications, expansions,
abstracts, additions, suppressions, transpositions) which the plagiarist
multiplies in order to throw suspicion off the scent The two guilty ones
are sufficiently betrayed by their common errors; the more culpable of
the two is detected by the slips he will have made, and especially by
the errors in his own papers which are due to peculiarities in those of
his accommodating friend. Similarly when two ancient documents are in
question: when the author of one has copied directly from the other, the
filiation is generally ea