1. Only one reading can be original, however many variant readings there may be.
2. Only the readings which best satisfies the requirements of both external and internal criteria can be original.
3. Criticism of the text must always begin from the evidence of the manuscript tradition and only afterward turn to a consideration of internal criteria.
4. Internal criteria (the context of the passage, its style and vocabulary, the theological environment of the author, etc.) can never be the sole basis for a critical decision, especially when they stand in opposition to the external evidence.
5. The primary authority for a critical textual decision lies with the Greek manuscript tradition, with the version and Fathers serving no more than a supplementary and corroborative function, particularly in passages where their underlying Greek text cannot be reconstructed with absolute certainty.
6. Furthermore, manuscripts should be weighed, not counted, and the peculiar traits of each manuscript should be duly considered. However important the early papyri, or a particular uncial, or a minuscule may be, there is no single manuscript or group or manuscripts that can be followed mechanically, even though certain combinations of witnesses may deserve a greater degree of confidence than others. Rather, decisions in textual criticism must be worked out afresh, passage by passage (the local principle).
7. The principle that the original reading may be found in any single manuscript or version when it stands alone or nearly alone is only a theoretical possibility. Any form of eclecticism which accepts this principle will hardly succeed in establishing the original text of the New Testament; it will only confirm the view of the text which it presupposes.
8. The reconstruction of a stemma of readings for each variant (the genealogical principle) is an extremely important device, because the reading which can most easily explain the derivation of the other forms is itself most likely the original.
9. Variants must never be treated in isolation, but always considered in the context of the tradition. Otherwise there is too great a danger of reconstructing a “test tube text” which never existed at any time or place.
10. There is truth in the maxim: lectio difficilior lectio potior (“the more difficult reading is the more probable reading”). But this principle must not be taken too mechanically, with the most difficult reading (lectio difficilima) adopted as original simply because of its degree of difficulty.
11. The venerable maxim lectio brevior lectio potior (“the shorter reading is the more probable reading”) is certainly right in many instances. But here again the principle cannot be applied mechanically.
12. A constantly maintained familiarity with New Testament manuscripts themselves is the best training for textual criticism. In textual criticism the pure theoretician has often done more harm than good.
Kurt Aland and Barbara Aland, The Text of the New Testament, pp. 275-276.