In 1885, Friedrich Maassen showed that the Collectio Hispana Gallica which serves as a vessel for the False Decretals was no ordinary version of this collection, but rather a special Pseudo-Isidorian recension, complete with inauthentic adulterations. He also showed that this adulterated Pseudo-Isidorian Hispana survives mostly or entirely separately from the decretal forgeries, in Vatican, Biblioteca Apostolica, Vat. lat. 1341. Maassen himself and others since have been fond of writing that Pseudo-Isidore interpolated the Hispana by way of preparing it to receive his decretal forgeries. I too am guilty of writing this way. The problem with this view is that it makes no sense.
First of all, we must ask ourselves: Why did the Hispana have to be prepared at all? If indeed its contents required revisions, these could have been carried out alongside the more major changes, that is to say the massive expansion of the authentic Hispana decretals with forgeries. Not only are we supposed to think that Pseudo-Isidore wasted time and parchment producing a fair copy of this lightly retouched collection; he or somebody else also took steps to circulate it. Only one complete copy of the interpolated Hispana survives today, but even if we confine ourselves to the ninth century we have evidence of two further copies (the initial folios of Vat. lat. 630 and the s. IX med. additions to Berlin, Hamilton 132). In total, six or seven medieval copies of the Pseudo-Isidorian Hispana have left traces.
Second, the interpolations to the Hispana do not really mirror the agenda of the decretals forger. Instead they address a variety of miscellaneous concerns, in much the same way as the False Capitularies of Benedictus Levita tend to do. The decretal forgeries stand apart from both in presenting far less of a miscellany, with a much tighter focus on procedural matters.
What can this mean? In a forthcoming article for Deutsches Archiv, I show with many tables and parallel columns and so forth why all three books of the False Capitularies, together with the first three Additiones, must predate the False Decretals. So we began to get an earlier picture, of miscellaneous concerns in the Hispana interpolations and in the capitularies; and a later picture, of increasing preoccupation with matters of procedural law.
In fact, and just as an aside, you can by and large determine the relative chronology of Pseudo-Isidorian items by their degree of procedural hysteria. Take for example the Capitula Angilramni: There are strong source-critical reasons to assign them to the same era as Book III of Benedictus Levita and just prior to the False Decretals. They focus exclusively on procedural protections for bishops. The same is true of Additio IV of Benedictus Levita, which was assembled very late, after drafts of at least some decretal forgeries had been drawn up. Early items in the Pseudo-Isidorian library, however, like Benedictus Levita Book I and Book II, have some of the procedural material, but they balance it against various other concerns.
Third, further on the matter of chronology and its implications. It is plain to me that Pseudo-Isidore revised the Hispana at the earliest stages of his project, alongside Book I of Benedictus Levita. At one point early on in Book I, Benedictus is forced to resolve a textual problem with a decretal of Leo transmitted to him in corrupt form by the Hispana. The Hispana interpolator encounters the same corruption and applies a different, independent solution. Conclusion: Benedictus had before him the unredacted Hispana Gallica and not yet the Pseudo-Isidorian recension.
By the time Benedict got to Book II, however, the interpolated Hispana was more or less finished, because Book II includes unmistakable excerpts from it. These excerpts are extensive, so much so that they allow some very precise conclusions about what the Benedictus Hispana looked like; it was slightly different from what we have. After Book II, the forgers still had to compile Book III of the False Capitularies, the most complex stretch of the capitulary forgeries by far, where procedural concerns come to the fore more and more. Then they had to assemble the Capitula Angilramni, and only then the decretal forgeries. These were then finally embedded in the Hispana.
The distance between the interpolation fo the Hispana and the forging of the decretals is thus very great. The former is not immediately prior or preparatory to the latter.
In sum: Nobody has explained why Pseudo-Isidore needed to prepare the Hispana in the first place; the adulterated Hispana circulated on its own and was not merely an internal draft; the Hispana interpolations align with early Pseudo-Isidorian concerns and do not have the developed procedural hysteria of the False Decretals; and in fact the interpolated Hispana was produced well before the decretal forgeries.
There is a powerful case to made that minor items like the Capitula Angilramni were indeed preparatory to the False Decretals. The interpolated Hispana is totally different, and we are driven to ask why Pseudo-Isidore bothered with it at all.