Lectio difficilior potior (Latin for "the more difficult reading is the stronger") is a main principle of textual criticism. Where different manuscripts conflict on a particular word, the principle suggests that the more unusual one is more likely the original. The presupposition is that scribes would more often replace odd words and hard sayings with more familiar and less controversial ones, than vice versa. It will readily be seen that lectio difficilior potior is an internal criterion, which is independent of criteria for evaluating the manuscript in which it is found, and that it is as applicable to manuscripts of a roman courtois or a classical poet as it is to a biblical text.
The principle was one among a number that became established in early eighteenth-century text criticism, as part of attempts by scholars of the Enlightenment to provide a neutral basis for discovering an urtext, independent of the weight of traditional authority. The principle was first laid down by Johann Albrecht Bengel, in his Prodromus Novi Testamenti Graeci Rectè Cautèque Adornandi, 1725, and employed in his Novum Testamentum Graecum, 1734. It was widely promulgated by Johann Jakob Wettstein, to whom it is often attributed.
Many scholars considered the employment of lectio difficilior potior such an objective criterion that it would override other evaluative considerations. The poet and scholar A. E. Housman challenged such reactive applications in 1922, in the provocatively titled article "The Application of Thought to Textual Criticism".
On the other hand, taken as an axiom, the principle lectio difficilior produces an eclectic text rather than one based on a history of manuscript transmission. "Modern eclectic praxis operates on a variant unit basis without any apparent consideration of the consequences", Maurice Robinson has warned, suggesting that to the principle, "the reading which would be more difficult as a scribal creation is to be preferred should be added a corollary, difficult readings created by individual scribes do not tend to perpetuate in any significant degree within transmissional history".
Robinson, as a noted proponent of the superiority of the Byzantine text-type, the form of the Greek New Testament that is found in the largest number of surviving manuscripts, would use this corollary to explain differences from the Majority text as scribal errors which were not perpetuated because they were known to be errant or that existed only in a small number of manuscripts at the time. The majority of textual-critical scholars would explain the corollary by the assumption that scribes tended to "correct" harder readings, and thus cut off the stream of transmission, so that earlier manuscripts would have the harder readings and later ones would not; hence they would not see the corollary principle as being a very important one for bringing us closer to the original form of the text.
However, lectio difficilior is not to be taken as an absolute rule either, but as a general guideline: "In general the more difficult reading is to be preferred", is Bruce Metzger's reservation. "There is truth in the maxim: lectio difficilior lectio potior ('the more difficult reading is the more probable reading')", write Kurt and Barbara Aland. But for scholars like Kurt Aland, who follow a path of reasoned eclecticism that is based on evidence both internal and external to the manuscripts, "this principle must not be taken too mechanically, with the most difficult reading (lectio difficillima) adopted as original simply because of its degree of difficulty". And Martin L. West cautions, "When we choose the 'more difficult reading'...we must be sure that it is in itself a plausible reading. The principle should not be used in support of dubious syntax, or phrasing that it would not have been natural for the author to use. There is an important difference between a more difficult reading and a more unlikely reading".