From Harmony this universal frame began*

Boethius playing the monochord
Anonymous,
Cambridge, University Library
Public Domain
Link

“Pythagoras and his followers devoted a great deal of attention to acoustical and musical phenomena. They regarded consonances—especially of a fourth, fifth, and octave—as models of that harmony, conceived of as an accord or equilibrium of different elements, which they equated with the human soul or with the ordering principle of the universe. The assignment of the numerical ratios that are at the basis of musical concordances was for the Pythagoreans the starting point for discovering the laws which governed both the feelings of the soul and the movements of the universe. They arrived at these results experimentally, through the monochord, whose invention was attributed to Pythagoras himself.”

 ~Music in Greek and Roman Culture, by Giovanni Comotti (tr. Rosaria V. Munson)

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*I just realized I’ve never shared John Dryden’s “Song for St. Cecilia’s Day” here, which is a horrible oversight. Do read it—it’s so beautiful and fairly sums up the medieval understanding of how the order of the entire cosmos is a musical relationship.