Ficino’s astrological music [which served as the basis for Holst’s The Planets] was based solidly in the Boethian notion of the essential congruity between the music of the spheres, musica mundana, the music of the human organism, musica humana, and ordinary music making, musica instrumentalis. That congruity, an aspect of the sympathetic relationship that exists between the human soul and the cosmic spirit (and which is reflected in the perfect proportions of instrumental music) causes like bodies to act in concert—in just the way that an open string of a lute, if it is in harmony with another string that is plucked, will vibrate sympathetically. This affinity, Ficino tells us, also explains why the human organism responds so profoundly to music. When musica instrumentalis is agitated, the soul of the person who hears it becomes agitated; when a composition is tranquil, the soul is tranquilized. An exactly parallel relationship exists between man and the cosmos: when a person’s soul is in tune with the heavens, it responds just as sensitively to the music of the spheres. Thus, for example, if Mars is the dominant planet, the ‘music’ of the soul will be harsh and martial; if it is Venus, then the soul will be voluptuous and delight in love.
Jamie James, “The Music of the Spheres” pg. 121-22