One of the strictly musical reasons I love medieval music and golden age polyphony is that it is modal. Being modal, it is frequently surprising. A subtle kind of variety existed before the Great Settling of Western music into the major/minor system. One sees that process of settling occur, frequently, in the treatment of half-steps, the disappearance of the Lydian, the emerging consensus about what constitutes an authentic cadence, etc. But who can resist, really, feeling affinity for a time when these things were not yet settled?

Is there any particular reason they can't be un-settled? I don't think so. Knud Jeppeson wrote a delightful book in 1939 about sixteenth century vocal counterpoint. In it, he lays out a very neat description of what I've long felt myself but never systematized:

A modern ear, accustomed to hearing a half step between the seventh and eighth degrees of the scale, feels a peculiar charm in the fact that the ecclesiastical modes do not use one single definite interval at this place, but use sometimes a whole and sometimes a half step. Likewise the free use of the B-flat for melodic reasons and the raising of the third of certain definite triads, which occurs with equal freedom, produce harmonic variations which sound refreshing and strange.

Fair enough, you say, yawn, etc. But here's the really interesting and surprising bit:

Every ecclesiastical mode actually has at its disposal many more chordal possibilities than the major and minor scales. If we compare, for example, the Dorian with D minor (the two having almost the ssame scale) we see that the Dorian has two triads (D major and D minor) on the first degree, whereas the D minor scale has only one. On the second degree there are two possibilities available for the ecclesiastical mode, but only one for the D minor scale.

The following illustration shows the resources ordinarily available:

Dorian: i and I, ii and ii(dim), III, IV and iv, V and v, VI and vi(dim), VII and vii(dim)
D minor: i, ii(dim), III, iv, V, VI, vii(dim)

As may be seen, we have in the Dorian no less than six pure triads -- the most valuable tonal combinations -- which deviate from D minor: D major, E minor, F major, G major, A minor, and C major.

In addition to the greater richness which the the particular ecclesiastical modes thus possess within their respective fields, they have the greater number of modes. While modern music only has two modes...

And he goes on. He does of course grant the major/minor system's superiority in the matter of potential for modulation. I wouldn't go that far myself because I think what can happen in the modal universe is a natural tendency toward "tonicizations," i.e. temporary tonics, that in the sixteenth century were temporary but needn't be -- they could be prolonged, and the effect could be greater.

What intrigues me most however is this beautifully subtle motion in the old, unsettled system between major and minor sounds, the subtle play of harmonic color, and its relationship to text. With Messiaen's symmetrical modes, even more color is possible.

Is it really impossible to imagine a sacred music for the future that returns to earlier procedures but with an expanded palette? What sacred music these days is like that? What can we learn from these sixteenth century masters?