Art and Beauty in the Middle Ages by Umberto Eco
Quite often the middle ages are the forgotten link between the Classical period and the Renaissance. Moderns assume that Renaissance thinkers and artists rediscovered the works of ancient Greece and Rome and had this amazing and original break with medieval tradition. Umberto Eco (author of The Name of the Rose among other works) dispels this myth with this overview of aesthetic and artistic theory in the years from AD 500 to 1400.
Eco shows the classical roots of medieval theory and theology. Platonic thought was dominant early on, where his ideal world of forms was the standard by which beauty and artistic craft were measured. Aristotle was rediscovered during the middle ages; his systematic approach was assimilated and imitated by the Scholastics. The centrality of symmetry and proportionality for beauty began with Pythagoras's focus on numbers. That focus was re-enforced and enhanced by the biblical notions that creation is good (cf. Genesis's account of creation) and that the world was made according to number, weight, and measure (cf. Wisdom 11). The world is both good and knowable in a mathematical way. It conforms naturally to the transcendental notion of beauty--that which is seen as good gives delight.
The medievals had much more to say about beauty than art. Beauty is a property all things have, both things in the natural world and things made by man. Often, artistic objects were judged beautiful by their symmetry to the world of ideals or of nature. Innovations in artistic theory were rare but not unprecedented. The theoretical trend followed the cultural trend--the great artistic achievements of the age were the cathedrals. They were built by many and varied artists whose anonymity was assumed. The work was done for God, not personal glory. By the late middle ages, individual artists were becoming more prevalent and theories such as nominalism (like Duns Scotus's notion of haecceity or "thisness" as the core of being, making individuality more significant than conformity to an ideal) embodied this shift of emphasis.
Eco does a fine job pulling together various sources from almost a thousand years of thought. He is able to distill a great portion of the history of philosophy and artistic consideration in a mere 120 pages. The book is very academic but is also accessible and a good introduction to aesthetics in the middle ages.