Internal Evidence of Readings is of two kinds, which cannot be too sharply distinguished from each other; appealing respectively to Intrinsic Probability, having reference to the author, and what may be called Transcriptional Probability, having reference to the copyists. In appealing to the first, we ask what an author is likely to have written: in appealing to the second, we ask what copyists are likely to have made him seem to write. Both of these kinds of evidence are alike in the strictest sense internal, since they are alike derived exclusively from comparison of the testimony delivered, no account being taken of any relative antecedent credibility of the actual witnesses.

B. F. Westcott and F. J. A. Hort, Introduction to the New Testament in the Original Greek with Notes on Selected Readings, 20.