gives the curve of a movement, we are able to accept their results without reserve, because we know by what method, by what instruments, by what system of notation they have obtained them. But when Tacitus says of the Germans, Arva per annos mutant, we do not know beforehand whether he took the right method to inform himself, nor even in what sense he used the words arva and mutant; to ascertain this a preliminary operation is required. This operation is internal criticism.
The object of criticism is to discover what in a document may be accepted as true. Now the document is only the final result of a long series of operations, on the details of which the author gives us no information. He had to observe or collect facts, to frame sentences, to write down words; and these operations,