Europeans began writing down music in the ninth century as an aspect of thepowerful orientation toward scriptuality that characterized the Carolingianculture. Whether as a matter of chance coincidence or not (I think not),medieval writing about music began, as far as we know, inthe same century and under the same cultural and political circumstances. Toget a sense of what the musically curious and informed thought should andcould be described and explained, we can consult the oldestcomprehensive—and most widely transmitted—didactic manualabout music that has come down to us from the Middle Ages, theMusica enchiriadis (“Handbook of Music”),written about 900 C.E. by an anonymous author.
[W]e can judge whether the construction of a melody is proper, anddistinguish the qualities of tones and modes and the other things ofthis art. Likewise we can adduce, on the basis of numbers, musicalintervals or the sounding together of pitches and give some explanationsof consonance and dissonance.
For a hint of what the author might have meant by “qualities,”in reference to music we can read a bit further:
[It] is necessary that the affects of the subjects thatare sung correspond to the effect of the song, so thatmelodies are peaceful in tranquil subjects, joyful in happy matters,somber in sad [ones], and harsh things are said or made to be expressedby harsh melodies [my emphases].
Regarding the association of “qualities” with “tones andmodes,” we can consult a manual written about two centuries later,the Micrologus by Guido of Arezzo. Guido writes that, forthe cognoscenti, recognizing the “characters and individualfeatures” of the modal patterns is like distinguishing people ofGreek, Spanish, Latin, German, and French origin from each other. Thus, the“broken leaps” of the authentic deuterus mode, the“voluptuousness” of the plagal tritus, the“garrulousness” of the authentic tetrardus, and the“suavity” of the plagal tetrardus are distinctly recognizable.Guido also writes that this diversity of characters matches the diversemental dispositions among different people, so that one prefers this mode,whereas another prefers that one.