Today we get to watch a musicologist in action; that is, we shall read his comments and ourselves comment upon them. This is taken from page 558 of Peter Williams' "The Organ Music of J. S. Bach." Commenting upon the prelude and fugue in G minor, he writes of the prelude:
"Only on paper could evidence be found for regarding this movement as an "Italian Courante' (Dietrich, 1931); neither the form (A B A B) nor the figuration (one harmony per bar, decorated) is typical of any courante. Clearly the conventional cadence formulae have been well learnt (bb 16, 22, 36) and the last might easily have been a phrygian half-close had it been conventional for prelude to en in this way. As elsewhere in the Eight, simple one-bar sequences above a basso continuo are so prominent as almost to suggest that their composer was consciously creating a series of samples."
The first comment, about the "Italian Courante" is directed at a previous scholar (one Dietrich, whose book dates from 1931, which should enable us to find it if we like and read his argument for ourselves). I have no idea what an Italian courante actually is. I have played a number of courantes, but I believe they are french, and though there are two distinct types (corrente and courante, one more active and the other more stately) I do not know about the Italian version, so I can't comment. His observation about the "one harmony per bar, decorated" is what is useful to our purpose.
Williams is pointing to the student-like nature of this piece. It is basically a series of root position chords (he makes that point elsewhere) filled in. What this means is that any reasonably competent music student, given a pattern of harmonies (and these really only follow the circle of fifths), could fill in the harmonies with arpeggiation--thus, it takes no genius to write something like this. The cadences, he notes, are well learned formulas. In fact, the paint-by-number compositional approach is so prevalent here (not badly done, in fact quite competent, just not very imaginative) that he thinks that the composer was creating "a series of samples." Samples?
This, oddly, is probably the one thing that could point us to Bach's authorship. The idea here being that the composer was simply trying to show simple technique in composition, i.e. providing a recipe to show students how to make a workmanlike prelude. I was recently reading a book by another musicologist who suggested the the famous C major prelude from the first book of the Well-Tempered Clavier, itself consisting entirely of a long chain of arrpegiated harmonies, was also intended as a student exercise.
But there is a major difference in quality between the two. Bach's C major prelude contains a number of very interesting voice leadings within those inner voices. Treating the entire measure of notes as one block chord (in fact, in the manuscript that's how they were written after the first five bars) one finds that there is a quite melodic nature of each of the individual voices within the chords, real counterpoint-- now one part moving up a note, now another down, to create a web of really ingenious harmonies. Within that web are a number of dissonances, for, as the notes move, they jostle against one another and create temporary disturbances. This is much more characteristic of a great musical mind; someone composing "by the book" will avoid things that "don't sound good" because they will not be thinking in large units of propulsion, but only of the actual musical moment before them.
In fact, I read once of someone using a computer to track the number of dissonances in known works of Bach and they found that there were a great many more than in any of his lesser known contemporaries. I believe the idea was to help in determining whether Bach actually wrote some of the pieces whose authorship is being debated.