Since 1937, critics have been discovering integrated into the structure of literary works various numerological and symmetrical patterns. Lines, chapters, books, episodes, characters, images, etc. are found in symbolic numbers like 3, 4, 10, 33, etc. and in symmetrical arrangements (concentric, triadic, or parallel). Both were known in classical times, but nowhere in surviving classical theory is there explicit recognition of any large-scale use of number or pattern in literature; nor are there important references in modern critical writing before this century. Nevertheless, even though the esthetic relevance of highly complicated or esoteric number systems is doubtful, many symmetrical (often concentric) and numerological patterns have been convincingly demonstrated in works from the Iliad to the Divine Comedy and Paradise Lost, as well as in other poems and novels from the Renaissance to the present. But problems remain as to what can properly be measured and what validates the patterns detected.