I find it particularly difficult to teach a section on Medieval Aesthetics in the Introduction to Aesthetics course. I have used the online encyclopedia of philosophy article by Michael R. Spicher, which is helpful. He begins with the influence of Plato, Aristotle and Plotinus, which is appropriate. I would only add that Plato's Symposium was particularly important, especially by way of its influence on Plotinus. I also like Spicher's division between three topics in medieval aesthetics: proportion, light and color and symbolism. Proportion is particularly relevant to architecture. I would add Pythagoras to the original list of influential philosophers since Medieval aesthetics is so strongly influenced by such concepts as harmony, symmetry and proportion. In teaching the material I decided to start with Diotima's description of the higher mysteries, i.e. the ladder of love. This fits in well with a description of Chartres cathedral. I was able to use a video from Kahn Academy which shows the stained glass windows of Chartres quite well. The only thing I disagree with there is that the writers say that the effect of the windows has nothing to do with aesthetics and only with divine symbolism. I would think that divine symbolism is one style of aesthetics and, generally, I object to using the term "aesthetic" only to refer to superficial attractiveness. The section on "Light and Color" in Spicher's article particularly relates to Chartres, although one might also have to explain the Neoplatonic theory of emanation. One thing that we see in medieval aesthetics that we do not see in Plato and Aristotle is stress played on both color and radiance. Plato brings in light when dealing with the allegory of the Sun, but doesn't seem concerned with color. I see the Medieval interest in color as an anti-dualist moment or aspect of Medieval thought: they are asking that we pay attention to beauty in sensuously rich experience in a way that Plato would not. This also relates to the Medieval ideas of radiance and clarity. Plato does speak of beauty as a vast sea, but he does not see that beauty in terms of any special notion of radiance. It is not that the Medievals believe that God is Light, as Spicher implies, but that God is symbolized in a deep way by light especially insofar as it seems to emanate from the things themselves. Plotinus provides a transition from Plato to the Medievals: Spicher quotes him "The simple beauty of a color is derived from a form that dominates the obscurity of matter and from the presence of an incorporeal light that is reason and idea." (1.6) I love the quote from Hugh of Saint Victor, also found in Spicher: "With regard to the color of things....sight itself demonstrates how much Beauty it adds to nature, when this last is adorned by many different colors." Spicher puts it in an interesting way: "There is a sense in which color causes beauty, since everything has color. Hence, more radiant colors will cause the object to be more radiant and, therefore, more beautiful." Of course this could be taken too literally. Spicher takes Symbolism to be a third elemental, although I find it hard to separate this from the issue of radiance. If the world is a divine work of art then it will be radiant. In my own thinking, even in a world without God, there is radiance of the world, and this could lead to seeing art as pointing out and enhancing this radiance, the radiance of everyday life. This could be related to the Christian view of hermeneutics. If beauty is a reflection of God's beauty and if, as Aquinas held, all knowledge about God begins inn the material realm through the senses (this is how Spicher puts it) then one way to see beauty is radiance that comes from things seen being full of meaning. Nothing I am saying, of course, is fully consistent with Medieval aesthetics: it could not be, since I am coming from an atheist standpoint.
To continue, I find particularly valuable the notion of radiance. Spicher quotes Gilson "Radiance belongs to being considered precisely as beautiful: it is, in being, that which catches the eye, or the ear, or the mind, and makes us want to perceive it again." Spicher writes: "Radiance signifies the luminosity that emanates from a beautiful object, which initiatlly seizes the attention of the beholder." For Aquinas "All form, through which things have being, is a certain participation in the divine clarity [or light]. And...particulars are beautiful because of their own nature - that is, because of their form."
Bottom line for me: the Medievals made one important contribution to aesthetics, i.e. the notion of beauty as radiance that is full of meaning. I have spoken of this as "aura" in my book The Extraordinary in the Ordinary: The Aesthetics of Everyday Life.