Before I begin, let me explain the fundamental idea behind Thomist aesthetics. According to Thomism unity, goodness, truth, and beauty are all transcendentals. Roughly, this means that wherever we find being, we also find unity, goodness, truth, and beauty, because all of these concepts point to one aspect of the notion of being. Thomist aesthetics, unlike other theories, begins with being, and not with a formalized and restrictive definition of beauty or art.
The naive aesthetic theory that I will call “realism” holds that art is an imitation of something real. This makes sense when looking at much of classical art and the history of painting, but utterly fails to explain why an impressionist or expressionist painting (by Claude Monet and Julius Evola, respectively) is considered good, despite that fact that art from the impressionist and early expressionist periods is almost universally loved by non-specialists. Similarly, a popular theory by Arthur Danto, often called “institutionalism” claims that art is whatever artists, galleries, and art-institutions can get away with calling 'art'. This definition seems to bypass the problem we are discussing, namely being able to comprehend a variety of different art styles, but it does so by ignoring the problem, not by answering it. This answer is unsatisfactory, because we want to know why some art seems good, beautiful, or satisfying, while other art does not. These two trends of realism and institutionalism represent most attempts at aesthetic theorizing. Theories that tend towards realism look for some objective feature of art that relates to the world, and all art can then be judged by whatever standard is put forward as the essence of art. Theories that tend towards institutionalism make all aesthetic judgment subjective by saying art is merely a sociological category, or is merely the cause of a subjective emotional response. Thomist aesthetics finds the middle road between these two paths by pointing to being as the foundation of beauty. By recognizing that beauty is an inherent aspect of being, Thomists can make sense of classical realistic art, an impressionist painting, and an abstract expressionist painting.
In a realistic painting or a portrait, a clear depiction of an object or person is the being dsplayed. For example, a young shepherdess is the subject of this painting by William Adolphe Bouguereau, and it is her beauty that is grasped, through her being, when we see the painting. In an impressionistic painting, like this one by Erin Hanson, what is portrayed is a subjective impression of some scene or object. According to Thomism, however, our feelings, emotions, and experiences all have being, and it is this being that is displayed in impressionist paintings. We can follow this logic even further to make sense of an abstract painting that consists two red and blue triangles, or even a painting consisting of one single black line, both by artist Ellsworth Kelly. Triangles and lines have a type of being (often called 'conceptual being' by Thomists), although it is not the exact type of being encountered in physical reality. In abstract paintings like these what is being displayed is the conceptual being of abstract objects contemplated in our minds. The beauty of modern art consists of the beauty contained in conceptual being. It may even be useful to think of the beauty of modern abstract art as more akin to the beauty one finds in the truth of a well-formed, true syllogism or a mathematical formula than to the beauty of a realistic portrait. But no matter how we think of it, it is beauty nonetheless.
Whatever its other virtues or vices as a theory, Thomist aesthetic theory manages to explain a vast array of different types of art, and for this reason alone it should be considered a strong and interesting theory, well worth the time of those looking into aesthetics.