The object of all textual criticism is to recover so far as possible the actual words written by the writer. But in order to do this properly the critic has to explain how each successive deviation from the original came to be currently adopted, and frequently he finds the clue enabling him to do this in the history of some later period, which gives some reason for a textual variation.

Kirsopp Lake, The Text of the New Testament, 4th ed., rev., (New York: Edwin S. Gorham, 1908), 1.