number of attesting documents must be treated as incommensurable,
C. 58, 59. Manner of discovering genealogy
58. Knowledge of the Genealogy of Documents, as of other facts respecting them, can sometimes be obtained to a certain extent from external sources, under which may be included various external indications furnished by themselves; but it is chiefly gained by study of their texts in comparison with each other. The process depends on the principle that identity of reading implies identity of origin. Strictly speaking it implies either identity of origin or accidental coincidence, no third alternative being possible. Accidental coincidences do occur, and have to be reckoned for: but except where an alteration is very plausible and tempting, the chance that two transcribers have made the same alteration independently is relatively small, in the case of three it is much smaller, and so on with rapidly increasing improbability. Hence, while a certain number of identities of reading have to be neglected as capable of either interpretation, the great bulk may at once be taken as certain evidence of a common origin. Such community of origin for a reading may of course as regards the two or more attesting documents be either complete, that is, due to a common ancestry for their whole texts, or partial, that is, due to ‘mixture’, which is virtually the engrafting of occasional or partial community of ancestry upon predominantly independent descent.
59. Here, as in the investigation of the comparative excellences of continuous texts, we are able to arrive at general conclusions about texts by putting together
