"Works of art make rules. Rules do not make works of art."
-- Claude Debussy
When we're studying the scales we learn a firm rule very early on that the melodic minor used when writing moving lines is to have scale degrees 6 and 7 sharpened in ascending (the reason being, the augmented 2nd interval between 6 and 7 of the harmonic minor is hard to sing going upward -- plus the scale needs a leading tone as the tonic note is approached) and in descending it will follow the key signature of the passage in which it's written, i.e., with no additionally sharpened 6th or 7th.
We get this firmly into our heads and think we have it all figured out from then on.
And then comes a day when the passage we've written in our fugue has an ascending minor scale and so we sharpen 6, only to discover that this creates an intolerable dissonance with one or more other moving lines, a dissonance which disappears however when we use the harmonic minor, i.e. without a sharpened 6th.
And now we've switched out the melodic minor for the harmonic minor, broken a "rule" in the interests of art, eliminated an intolerable dissonance thereby, and arrived at a harmonious outcome.
We've learned an important lesson from this -- that in the minor key we have to look at what's going on all around an ascending line before we settle on its exact pitches, and what works in the vast majority of applications may not always, not 100 per cent of the time.
In the score for the G Major Op. 31 Postlude we find just such a situation, an example where the entries in minor keys incorporate ascending lines in harmonic minor; this is to eliminate the intolerable dissonances which would be created by following the rule of melodic minor.
No rule in organ playing is every "absolute;" there is no such word in the glossary of organ playing, and the same applies to organ composition.
The rule book is and always has been a friend -- a system of common practice derived from the successful workings of a vast number of Western composers on back through many centuries of music writing.
The rule book then, as thus defined, is a "map" of sorts ... something in place to guide and serve composers of the present day, not to intimidate them, not to enslave them, not to be poised ready to crush them in an instant for the slightest deviations.
In the case of organ composition, composers first learn the rules and learn to live within the limits they impose so that, if the opportunity ever happens to present itself, they will know how to bend or break one of those rules like an artist.
It's a provable fact that no composer of eminence ever arrived at a level of ultimate mastery without having passed through a period of strict discipline with respect to the rules of composition and part writing ... just the sort of thing one would find laid out in the rule book.
It's a provable fact that the artistic bending or breaking of a rule, at times, can lead a composer to discover beauty where they were told, or led to believe, that there was none [See blog, Monotony, Getting Started With Writing, Part XX].
The Bach Chorales are a perfect example of this; we keep returning to Bach to illustrate various points because a very strong argument can be made that, of all composers, he's the undisputed master of part writing.
Bach composed entirely from the mind away from the instrument and demanded the same from his scholars; the reason for this is obvious -- it allows the voice eleading to proceed with complete freedom and independence.
This leaves a trail of part writing that's all over the map.