A sufficient reason for a new edition of Roger Bacon's principal work would be the extreme rarity of the edition of the Opus Majus published by Jebb in 1733, and reprinted seventeen years afterwards in Venice. But a more cogent reason is that this edition is incomplete. The work, as we learn from Bacon's account of it in his Opus Tertium, consisted of seven parts; and the seventh part, a discourse on Moral Philosophy, was omitted by the editor.

Why Jebb should have taken this course is not clear. In his preface he speaks of the work as consisting of six parts, ‘in sex partes distributum,’ and adds, ‘tractatum de Morali Philosophia ad calcem adjunxit.’ In 1858 a paper was read by Dr. Ingram before the Royal Irish Academy, and was printed in the seventh volume of the Proceedings of this institution, in which the writer showed conclusively the continuity of this seventh part of the Opus Majus with all that had gone before. The continuity is marked unmistakably in the very title of the section, Incipit septima pars hujus persuasionis de Morali Philosophia, and in its opening words, ‘Manifestavi in praecedentibus,’ &c. Repeated references to the foregoing parts will be found; and if further proof were wanting, it is supplied in abundance by the two appendages to the Opus Majus which were sent by Bacon to Pope Clement IV within a few months of the dispatch of the principal work, published by Professor Brewer in 1859, in the Rolls Series, as Opera Inedita.