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 reading, 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. 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, a classical poet, or a Sanskrit epic as it is to a biblical text.

The principle was one among a number that became established in early 18th-century text criticism, as part of attempts by scholars of the Enlightenment to provide a neutral basis for discovering an urtext that was independent of the weight of traditional authority.

Rabbeinu Tam (1100 – 1171) expressed the idea in his work 'Sefer Hayashar': "ובעל התלמוד כתבו, שתלמידים המגיהים אינם מגיהים דברים של תימה" ("it was written by the author of the Talmud, since students who correct the text do not correct it in order to make the text difficult", responsum 44). Erasmus expressed the idea in his Annotations to the New Testament in the early 1500s: "And whenever the Fathers report that there is a variant reading, that one always appears to me to be more esteemed (by them is the one) which at first glance seems the more absurd-since it is reasonable that a reader who is either not very learned or not very attentive was offended by the specter of absurdity and changed the text."

According to Paolo Trovato, who cites as source Sebastiano Timpanaro, the principle was first codified by Jean Leclerc in 1696 in his Ars critica. It was also laid down by Johann Albrecht Bengel, as "proclivi scriptioni praestat ardua", 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 an objective criterion that would even 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 A. Robinson warned. He suggested that to the principle "should be added a corollary, difficult readings created by individual scribes do not tend to perpetuate in any significant degree within transmissional history".

A noted proponent of the superiority of the Byzantine text-type, the form of the Greek New Testament in the largest number of surviving manuscripts, Robinson would use the corollary to explain differences from the Majority Text as scribal errors that were not perpetuated because they were known to be errant or because they existed only in a small number of manuscripts at the time.

Most textual-critical scholars would explain the corollary by the assumption that scribes tended to "correct" harder readings and so cut off the stream of transmission. Thus, only earlier manuscripts would have the harder readings. Later manuscripts would not see the corollary principle as being a very important one to get closer to the original form of the text.