Since I started reading the decretals, I've tried to get my head around Pseudo-Isidore's basic unit of composition. In a very early post I asserted that our forgers "were in thrall to [their] sources," and said that most of the decretals don't have much consistency or coherency beyond the level of the paragraph.
I don't think that's wrong, exactly, but I've gradually become aware that these forgeries operate at a broader level too. Let's take the two Cornelius decretals (here and here) as an example: I just blogged them so they're fresh in my mind.
The first one doesn't say all that much. Aside from some general pastoral pablum taken from a letter of Pope Martin I,* it basically just expands on a few statements in Corenlius's Liber Pontificalis biography. The second Cornelius letter, though, has some substantial points to make: on oaths, accusations by inferiors, peregrina iudicia, and absentee trials. We said before that Pseudo-Isidore likes his letters to have a historical hook wherever possible. He likes to forge letters that show his popes doing what other sources -- primarily the Liber Pontificalis -- say they did. This gives the letters a patina of authenticity, and integrates them -- however superficially -- with the historical record. How our forgers can devote so many words to this rather cunning project on the one hand, while undermining it with silly anachronisms on the other must remain a mystery.
Now take another look at those Cornelius letters. It's clear that Pseudo-Isidore has marked out one (namely the first) for his historical hook, and reserved all of his hard content for the second. And as I page through all the stuff I've read so far, I realize that this tactic is not uncommon: Often when you have a letter that's pretty light on content, it turns out to be one of two or three ascribed to a given pope. Pseudo-Isidore has just stashed all his arguments in the other epistles. When a given pope gets only one letter, on the other hand, you often have the historical hook and the content side-by-side.
Now this may be a good general rule, but isn't always true. Urban I is one contrary example. This is one of those popes who gets only one letter, and his has nothing but content and no historical hook at all. At the same time, it's a really unusual letter that breaks the mold in other ways as well -- it's the one that trots out the novel principle that estates should simply be ceded to the church, and not sold to generate alms, as per apostolic example.
Anyhow. The idea that the content may have been planned and developed pope-by-pope (rather than, say, decretal-by-decretal, or paragraph-by-paragraph) seems worth looking into.
* I hasten to add that words taken from the mouth of one pope and put in the mouth of another, are not always, ipso facto, filler. Pseudo-Isidore is trying to build the impression of an abundant, unanimous, and cohesive tradition, after all. Sometimes he accomplishes this by taking something one guy said and ascribing it to a random selection of his predecessors.