Metaphysics of the Music, part 20
As to what it is in the beautiful object that is responsible for eliciting the affective response in the individual, Thomas lists the three objective properties of beauty—integrity, proportion, and clarity—touched on at the end of the last chapter.[1] By integrity or perfection, Thomas means something close to what Tolkien has in mind in his requirement that sub-created, secondary world’s exhibit the “inner consistency of reality.” Integrity, in other words, refers to a thing’s completeness, wholeness, or togetherness, its having and displaying the structure and requisite parts proper to a thing of that particular essence or nature. The second aesthetic property of proportion, sometimes referred to as harmony or consonance, is of Pythagorean extraction and designates in Thomas’s usage a sense of qualitative proportion that he calls convenientia, or what Liberato Santoro-Brienza describes as an “intrinsic attunement” or “correspondence between inner and outer reality, appearance and essence, matter and form.”[2] It is the harmony of beauty, in other words, that is especially involved the aforementioned, pleasing correspondence obtaining both between the parts and metaphysical principles within the object and between the object and the sensory faculties of the perceiver.[3] Finally, there is the aesthetic criterion of clarity, or brightness or “splendor,” i.e., the shining forth of form or radiating of intelligible light from the beautiful object, an idea that Robert Wood suggests is further linked to the Greek word doxa or “glory”[4] and which Tolkien gives us an apt image of in the Ainulindalë when the Ainur first glimpse the world, newly created by the Flame Imperishable of Ilúvatar, as “a cloud with a living heart of flame” (S 20).
[1] For an analysis of these properties in light of their development in Western thought prior to Aquinas, see chapter four of Eco, The Aesthetics of Thomas Aquines, “The Formal Criteria of Beauty,”64-121.
[2] Santoro-Brienza, “Art and Beauty,” 72.
[3] Robert Wood, Placing Aesthetics, 109.
[4] Wood, Placing Aesthetics, 105.
The Flame Imperishable 
Unfinished Tales by Tolkien was my initial introduction to the First Age, even before the Silmarillion. It sucked me right in.
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