The early Church fathers (hence "patristic," from the patriarchs) developed a system of interpretive delineation as a way to make sense of scripture: the Four Levels of Scriptural Exegesis. Medieval scholar D.W. Robertson, Jr., in the latter half of the 20th century, headed a movement to understand medieval literature and arts in terms of this system. (In Chaucer studies, the approach is often called Robertsonianism.) Robertson, somewhat fanatically, insisted that all medieval literature, and art, was understood, even composed, from this perspective by artists inevitably inculcated with exegetical skills. The medieval arts were generated from and created by artists steeped in this multi-tiered way of seeing and interpreting -- a sophisticated literacy which Western culture has subsequently lost.
THE LITERAL LEVELSince St. Paul himself insisted that "whatever was written ... was written for our instruction" (Romans 15.4), and since medieval artists seem to have taken to heart the essence of this assertion (e.g., Chaucer, NPT VII.3441-42, Retr. 1083), proponents of patristic exegesis argue that the creative products of craftsmen and artisans schooled in such an aesthetic are designed to function on multiple subtle interpretive levels. The most intriguing art is able "to mingle details of an iconographic nature with other details which produce an effect of considerable verisimilitude" (Robertson 242). Even if you don't buy into Robertson's extremism, it is useful -- often very productive -- to be thinking loosely at least on many levels of symbolism and significance when examining medieval art. The Unicorn Tapestries are good examples, but you will see others as well.In the most basic sense, an audience will enjoy a good story (e.g., the Old Testament tale of Jonah being swallowed by a whale -- or, actually, a big fish).THE TROPOLOGICAL LEVELFor a story to have any value, it is commonly expected to provide an edifying moral (e.g., as in the Jonah story, there is no escaping divine imperatives).THE ALLEGORICAL LEVELHere is where the Robertsonians and modern interpretation part ways. Patristic exegesis would insist that the stories prefigure even more crucial scriptural material (e.g., Jonah in the whale's belly allegorically anticipating Christ's descent into hell prior to his resurrection).THE ANAGOGICAL LEVELUltimately all stories illuminate heaven's divine plan and contain a message relevant to Christian spiritual salvation. Robertson was convinced that this message inevitably involved the doctrine of caritas -- Christian charity (e.g., the Jonah story finally denotes Christ's active message of redemption through his willing descent which served as a manifestation of divine charity).
Donaldson, E. Talbot. "Patristic Exegesis in the Criticism of Medieval Literature: The Opposition." Speaking of Chaucer. NY: W.W. Norton and Co., Inc., 1970. 134-153.
Robertson, D.W., Jr. A Preface to Chaucer. Princeton, NJ: Princeton University Press, 1962.