Anyone who has studied Roman law will have heard of the “Massentheorie” of Friedrich Bluhme, who discovered in 1818 that the excerpts from juristic literature assembled in the titles of Justinian's Digest tend to appear in accordance with a fixed sequence of works from which they derive. The same sequence is not found in every title – which is why no one had noticed the phenomenon before – but careful analysis showed that the same sequence is found within blocks or masses of material, and that it is the arrangement of the masses rather than of the works they contain that varies from title to title. There are exceptions to the general scheme, and works from which few excerpts were taken are harder to assign to particular masses than others, but there is little doubt that the ordering of works within three or four masses exists. On this basis Bluhme devised the theory that the commissioners charged with the task of compiling the Digest had been divided into three committees, that each committee had been assigned a list of works to examine (the fourth mass being formed from works that came to light at a later stage), that the works assigned were examined in the order discernible in the Digest, and that titles were eventually put together by placing one mass of material after another, generally with the largest mass first. The theory helped to explain how the Digest could have been compiled so quickly – two thousand books were processed in less than three years, according to Justinian – and it has now been widely accepted by scholars of Roman law for almost two centuries.

In the 1970s Tony Honoré published a series of articles and books in which he drew attention to a new discovery he had made about the Digest. Since the works of the jurists were made up of “books”, all consisting of about the same number of lines, and since the inscription at the start of each Digest text specified the particular book from which it was taken, Honoré realised that it would be possible to calculate the amount of material comprised in each mass and the rates at which excerpts had been taken from different works. The dramatic discovery he claimed to have made was that the works excerpted in the Digest fell into not only three or four masses but also two groups of equal size within each of the main masses. The rates at which the works of the jurists had been excerpted varied from group to group, and this finding, together with many other acute observations, enabled Honoré to devise a new theory. With initial assistance from Alan Rodger, he argued that Bluhme's committees had been divided into subcommittees, that groups of works had been assigned to each one to be processed in keeping with a strict timetable, and that a single commissioner had taken the lead in fulfilling the task of each subcommittee, calling for aid from its other members only if he felt the need to do so. The new theory further helped to explain the speed at which the Digest was compiled, but it did much more than that. Coupled with the analysis of writing styles, it promised to make it possible for the intervention of particular compilers to be identified. For the first time, it seemed, the identification of interpolations in the texts would have an objective foundation. Early adherents of the theory talked of a revolution in Roman law scholarship, and of a need to rewrite vast tracts of the subject.

It would be a considerable understatement to say that Honoré's publications provoked a lively debate. Given that scholars of Roman law were being challenged to rethink the basic methodology of their discipline, it was perhaps inevitable that those who were unconvinced would react with some violence. A tendency towards vituperation in reviews of his books was in danger of obscuring the seriousness with which argument was engaged in on either side, particularly in rigorous surveys of the numerical data that began to appear in the 1980s. The theory was far from receiving the widespread approval accorded to Bluhme's. More significantly, it was shown that Honoré's groups could not be claimed to have the same objective status as Bluhme's masses. The proposed division into equally sized groups of works was not the factual basis on which a theory could be built but was itself a matter of theory, for there were other ways in which the lists of works could be divided to reveal suggestive numerical patterns. Whether Honoré's division made sense was just as open to argument as his hypotheses about subcommittees, timetables and lead commissioners.

While most scholars would have found this level of criticism so disheartening that they diverted their energies elsewhere, Honoré is made of sterner stuff. He has continued to work on the sources and literature of Roman law in the past few decades, and in Justinian's Digest he has gathered together a series of studies on the theme that attracted criticism in the past. An introductory chapter is followed by a reworking of an article published in 2004 (121 Zeitschrift der Savigny-Stiftung für Rechtsgeschichte (romanistische Abteilung) 1–43) in which the theory devised in the 1970s is revisited, revised in the light of the rival surveys it stimulated, but adhered to in its essentials. The third chapter, published as a separate article in 2006 (3 Roman Legal Tradition 1–47), examines the minor works that cannot be placed with confidence in any particular mass – comprising no more than three per cent of the total – and is backed up with an “addendum” listing the works excerpted in the order in which they are now believed to have been read. The fourth chapter turns to the texts that appear to be out of sequence in the Digest – no more than seventeen per cent of the total – and reviews the findings of an article published in 1973 about the editing of the final compilation. This chapter is also backed up with an “addendum”, providing an analysis of all the displaced texts. The fifth chapter, developed from an essay published in a memorial volume for Peter Birks in 2006 (Mapping the Law 497–512), examines Bluhme's fourth mass, while the sixth chapter, developed from an essay published in a festschrift for Luigi Labruna in the following year (Fides, humanitas, ius 2529–42), examines several texts attributed to Paul that actually appear to have been the work of Ulpian. Now in his eighties, Honoré reflects that Justinian's Digest will probably contain his “final thoughts” on the topics discussed, though there is no sign that his powers are diminished. His ability to write with exceptional lucidity about extremely complicated matters, which impressed even his harshest critics in the 1970s – one remarked that an Honoré book is an ‘intellectual joy’ – remains very much in evidence.

The claims made for the revised version of the “Gruppentheorie” are relatively modest. That the works excerpted were not definitely divided into groups in the way proposed is clearly acknowledged (p. 21), as is an earlier tendency in dividing up the works to pay too much attention to the desire for numerical balance and not enough to their subject-matter (p. 68). That the imposition of a timetable on the commissioners was merely a “hypothesis” is similarly acknowledged (p. 18), that the commissioners generally adhered to the timetable is admitted to have been wrong (pp. 25–6), like the supposed allocation of the task of editing as well as excerpting to the three committees (p. 108), and that the views presented are “to some extent speculative” is conceded at the outset (p. 1). The claim now is that, “though hypothetical, the theory here reformulated can claim to be the most plausible” (p. 45), and that the theory still provides “the best explanation of the speed with which the task of reading and excerpting was completed” (p. 9). It is no longer imagined that a paradigm shift is under way in the study of Roman law, or that it will be incumbent on anyone about to undertake research in the subject to begin by forming a judgment about the validity of the theory. Instead, what Justinian's Digest offers is a contribution to a specific and increasingly specialised branch of research. The experts may not be convinced, and the rest of us may have our doubts and quibbles, but what can scarcely be denied is that thinking about the character and compilation of the Digest has been greatly enriched as a consequence of the stimulus Honoré provided forty years ago. A review of the current state of the debate by the scholar who set it in motion will be essential reading for anyone interested in these important topics.