Lectio difficilior potior: principle of textual criticism.
The Dutch humanist Desiderius Erasmus (1466-1536) was the first to introduce the principle of lectio difficilior potior to textual criticism. The Latin phrase means that a textual variant that is more difficult, is stronger.
Erasmus had discovered that copyists tended to make difficult texts simple; the opposite rarely happened. He deduced that if two manuscripts rendered the same section differently, the most difficult variant was to be preferred. (Of course this rule of the thumb only applies when both variants are equally logical and grammatically plausible, because otherwise, unusual expressions would be accepted as authentic only because they are difficult.)
The principle can be applied to texts, but also to their contents. An example is the pseudepigraphic text known as The Nine and a Half Tribes, which is mentioned in several old Christian texts. Other texts deal with the same subject matter, the fate of the lost tribes of Israel, but reckon them as ten. This is, of course, the more normal form. Since both groups of texts appear to be derived from the same original, it is likely that this mentioned nine and a half tribes; it is more logical that later authors changed 9½ into 10 than 10 into 9½.