 Eriugena’s major work, the Periphyseon, also titled in some manuscripts, De divisione naturae (On the Division of Nature), is a long dialogue between a “teacher” (Nutritor) and a “student” (Alumnus), was probably begun in the early 860s, just after he had completed the Pseudo-Dionysius translation, and finished around 867 (the date Wulfad, to whom the work is dedicated, became bishop, making it unlikely that Eriugena would have referred to him as frater, brother, after his consecration as bishop). The first printed edition of this dialogue appeared in 1681 and subsequently an edition by Floss appeared in 1853 which is now contained in PL 122. Inglis Patrick Sheldon-Williams (1908–1973) began a new edition of Periphyseon with the publication of his text and translation of Book One in 1968 in the Scriptores Latini Hiberniae series of the Dublin Institute of Advanced Studies, followed soon after by his edition of Periphyseon Book Two in 1970, and Book Three, published after Sheldon-Williams’ death, in the same series in 1981, seen through the press by the late John J. O’Meara. Sheldon-Williams had assembled materials for the edition of Books Four and Five and had completed a draft English translation of these books, which was published separately in one volume edited by John J. O’Meara. O’Meara and Édouard Jeauneau continued the edition of Sheldon-Williams, bringing out Book Four in the same series in 1995. However, due to some deficiencies in Sheldon-Williams’ manner of editing, and his mistaken conception of the manuscript tree, Jeauneau has undertaken an entirely new critical edition of the Periphyseon for the Corpus Christianorum Continuatio Mediaevalis series. All five books of the Periphyseon have appeared in this series. Sheldon-Williams was of the opinion that Periphyseon Book One had emerged from an earlier “essay in dialectic”, but offered no evidence for this interpretation, which takes an overly restrictive view of the first book of the dialogue. The main weakness of Sheldon-Williams’ editing strategy was that it had conflated various versions of the text drawn from several different manuscripts into a single composite script, in a similar manner to the earlier editors, Thomas Gale in 1681 and H.-J. Floss in 1853. Gale and Floss had published editions that combined into a single text both the text of the main body of the manuscript and the various marginal annotations in different hands. This composite version disguised the gradual evolution of the text and Jeauneau is of the opinion that this mixed type of edition is inadequate to the needs of scholarship. The new Jeauneau edition is based on six manuscripts, including two manuscripts, Paris Bibl. nat. lat. 12965 and Bamberg Phil. 2/2, not used by Sheldon-Williams since they contained only Books Four and Five. Jeauneau has suggested that the extant manuscripts of the Periphyseon show four distinct levels of development, that is, four early “editions”. One special difficulty in editing the Periphyseon is that the earliest manuscripts preserve only the first three books whereas the extant manuscripts for Books Four and Five date from the twelfth century. Avranches and Cambridge, both twelfth century manuscripts, are the sole witnesses for the end of Book Four and the whole of Book Five in Stage Two versions, with Avranches noticeably less accurate than Cambridge in several places.