All the longs must be equally long, all the shorts of equal brevity. . . .  In accordance with the length durations let there be formed short beats, so that they be neither more nor less, but one always twice as long as the other. (Commemoratio Brevis, early 10th cent.)

In the neumes it is necessary that you pay close attention where the proportional shorter duration is to be measured and where, on the contrary, the longer duration, lest you execute as quick and short what the authority of the masters has determined should be longer and more extended.  Nor should we heed those who say there is no reason whatsoever for our making now the quicker duration, now the more prolonged one, in a chant with a naturally disposed rhythm. (Berno of Reichenau, early 11th cent.)

In olden times great care was observed not only by the composers of the chant but also by the singers themselves to compose and sing proportionally.  But this idea has already been dead for a long time, even buried. (Aribo, late 11th cent.)

The original chant rhythm, intermingling variously long and short sounds, has yielded since the eleventh century to an equalistic execution that has robbed the rhythmic movement of much of its attractiveness and done away with numerous means of expression. (Peter Wagner, early 20th cent.)

Note: The following quotation concerning rhythmic nuances stands in stark contrast to the proportional rhythm described by the other authors.

The rhythmic value of the episema varies greatly, and lends itself to the expression of the most delicate nuances.  When it appears above a clivis, for instance, it may double the value of the first note, or again, it may indicate merely a delicate touch, or the least lingering of the voice.  This remark applies to all the rhythmic signs, whether they be modifications of the neume or additions to it, whether they be letters or other signs.  The reason is this: the sign, like the neume itself, is influenced by its position.  The character of the note to which it is attached, the place of the rhythmic sign in the neume, the relation of the neume to the text, the nature of the rhythm of which it forms a part, the movement and expression of the musical phrase as a whole—all these are the factors which either increase or diminish the value of the rhythmic sign. (Dom André Mocquereau, early 20th cent.)

As we have seen, Dom Mocquereau admits that there were mensuralists [i.e. advocates of measured or proportional rhythm] during the Gregorian centuries; it would be interesting if clear evidence could be cited to show that during the same period there were some who were not mensuralists. (Dom Gregory Murray, mid 20th cent.)

The interpretive particularities and the finesse of the notation gradually disappeared . . . .  Gregorian chant appeared to be, and in fact became, a “cantus planus,” that is, a chant free of all expressive values.  The term “plainchant,” which so often designates Gregorian chant today, should be discarded because it is based on a false premise. (Dom Eugène Cardine, mid 20th cent.)

The theory of nuances is not tenable . . . we are dealing with nuances of nuances, and that is of course nonsense.  Between the uncinus and the punctum different categories of note values must be intended.  The semiologists’ rejection of mensuralism must rest on a prejudice.  What is more, their theory is not very consistent. (Jan van Biezen, 21st cent.)

These Quotes in the Original Languages