"To establish the harmony that should be joined to it..."

It would seem, in Rameau's time, it was assumed chant would be harmonized. Despite changing styles, we have endured that assumption ever since, such that unaccompanied, pure chant is often described as sounding too "monastic", which is code for "barren". This has less to do with the "evolution" of sacred choral music, which of course grew out of chant, than with the rising dominance of secular idioms, instrumental music, and especially organ. So prevalent were these influences by Rameau's time that he apparently could not endure the thought of chant unharmonized. His whole musical world revolved around harmony, a priori. If I had my copy of Stravinsky's "Poetics of Music", I would quote his anti-evolutionary stance, his belief that music would be purified by distillation into a single line.

I don't recall reading the Rameau in school, but the portion of Kathy's quote strikes me as somewhat irksome, even if out of context:

Observe that since plain chant was composed at a time when good modulation was not yet understood, it sins continually against the natural order.

The (implied "chicken v egg") equation of this statement seems to rest upon the fulcrum of what is "understood." I would posit that Rameau's "natural order" is a construct of contemplation, otherwise a systematic outcome based upon specific observations.
I, for one, cannot and do not relegate "melody" as a quantifiable, scientific entity. The real sin is the proffering of an uninspired melody at the expense of those which were truly inspired.

All of us are as attuned to the modal as to the tonal world. Those of us who aren't there yet are getting there as fast as they can. Hence, the writings of Rameau are wildly eccentric to us. They are not eccentric to the very large numbers of modern music lovers who have but a peripheral interest in chant and choral music of the 'modal epoch'. (If, in fact, they like chant at all, it is precisely because it is, or seems, 'esoteric'.) Rameau's writings give theoretical grounding for what, to them, seems self-evident. I, myself, can remember that time in my life at which chant sounded strange and modal counterpoint sounded strange. I, though, am in that minority to whom this strangeness was highly attractive and, even, addictive. Today I can think tonally or modally at will and, equally at will, be totally defined by the one or the other. They are both possessed of limitless fields of fiori musicali, opulent and sumptuous sonorities. Indeed, it is as foolish to condemn the Brahms requiem for not being Tallis' lamentations of Jeremiah as it would be to do the reverse. Likewise, it would be a shallow, very shallow, mind that would condemn 'I know that my Redeemer liveth' (which requires its accompaniment) for not being Ad te levavi (which is self sufficient), or vice versa.

Rameau's assertions, and the theory that defines them are, indeed, self evident - self evident, that is, within a self-referential system. They are quite valid in reference to the western tonal system as it became somewhat implied in the late renaissance, evolved slowly through the baroque, and reached its climax in the classical and romantic eras.

Modal music is as irrelevant to this system as is kabuki music, or any of the panoply of other indigenous musical systems throughout world cultures. This 'happens' to be a fact which does not (or, certainly is not meant to) imply that musics that do not represent western tonality are somehow inferior. It doesn't and they aren't. These are not value judgments.

About melody, it shouldn't be too difficult to demonstrate that a modal melody, say Verbum supernum prodiens nec patris, is self sufficient and in no need of chordal underlay or complimenting second or third voices. Verbum supernum is self referential and complete unto itself. Anything added to it defaces it by degree. (This is why 'accompanied chant' is not only a sin, but an oxymoron.) Not so a melody such as Down Ampney (to choose but one 'tonal tune' at random). It is incomplete without the tonal underlay which it, in fact, implies (or which implies it!). It is not satisfying, or is not completely fulfilling, without its harmonious accompaniment and its complimentary alto, tenor, and bass. Where the modal tune does not by nature require a simultaneous voice or voices, the tonal tune not only implies them but is not complete without them. Indeed, it is derived from tonal harmony and is part and parcel of that structure! Rameau is absolutely correct - within and relevant to the tonal system. His error was in concluding that that system was universally valid and infallible - an error into which the 'modernists' of every age fall victim.

Whenever I sing solos I sing chant, that is, modal melody. I never sing a tonal melody unless I have three other voices or an organ... because it isn't 'all there' without them.

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(How many, let us say, can sing Veni creator spiritus without thinking 'harmony' in the background. I would venture to say that we all can do that and that we do do that. Such is the nature of the modal tune.
Now, try singing Darwall (or Abbot's Leigh) without thinking of the underlying harmony along with it. I dare say most will mentally supply the missing organ chords or choral voices. Such is the nature of the tonal tune - which is integral with an harmonic structure.)

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