Thomas More's quote spoke of music becoming increasingly able to stir the emotions it was trying to produce in the listener. It's fascinating that Paul Hillier can sight in Pärt's tintinnabuli style three subdivisions of increasing response "to the words in ever greater detail, with a subtlety and variation unimaginable in the earlier works".
His divisions are, characteristic of a musical analyst, unhelpful and impossibly abstruse. "...the earliest, like drawing with just pencil and paper (the works of the later 1970s to early 1980s); the second, transferring this knowledge to the medium of fresco (mid-1980s to early 1990s); the third (mid-1990s to the present), now painting in oils...". But I suppose I can't blame him - there's only so much that sleeve notes can handle.
I might question the primacy of More's original goal - that the utopia for music is its increasing ability to stir specific emotions - but I think we can begin to see that general trend in a scientific approach to music. Pärt, in the 1970s, "began to study Gregorian chant (not to copy it, but to imitate the timeless quality of its ebb and flow); he also studied medieval and renaissance polyphony, and became fascinated, too, by the sound of church bells." This is music that is not so much concentrated on the Romantic ideal of "spontaneous overflow of powerful feelings" in Wordsworth's words, but music understood as a science - a part of the Quadrivium that included arithmetic, geometry, and astronomy. Perhaps this is why, as Pärt's music develops as an "aesthetic" (according to Hillier), it becomes like More's utopia, "responding to the words in ever greater details".