Metaphysics of the Music, part 21
We can see, then, that while Thomas is certainly keen, in the words of G.B. Phelan, to give “due consideration for the rôle of the perceiving subject in the apprehension of the beautiful,” going so far as to define “the beautiful as having a necessary reference to a subject,” there is nevertheless an “intransigent objectivism” to his account of beauty. This leads us, finally, to a consideration of the existential realism lying beneath Thomas’s theory of being. Existentialism in this context refers to the insight—now widely regarded as one of if not the central principle of Thomas’s metaphysical thought—that the act of existence, signified by the Latin infinitive esse, is the “actuality of all acts and the perfection of all perfections.” As the existential Thomists of the twentieth century argued, in identifying a thing’s act of existence as the “act of acts,” Thomas inaugurated a veritable revolution in metaphysics and the theory of knowledge, at the heart of which was the realization that, contrary to the “essentialism” of much of the metaphysical tradition both before and after him, it is not a thing’s intelligible, contemplatable essence or form, but its actual, real, concrete existence in the world that, in the words of Armand Maurer,
holds the primary place in the order of being. While not neglecting other aspects of being, such as form and essence, St. Thomas offers a radically new interpretation of being by emphasizing its existential side. This was a decisive moment in the history of Western metaphysics, for St. Thomas was transforming previous Greek and mediaeval conceptions of being, which gave primary place to form.
 Phelan, “The Concept of Beauty in St. Thomas Aquinas,” 162.
 Maurer, “Introduction,” in Aquinas, On Being and Essence, trans. Maurer, 10.