The building was a sacred symbol, and every part had the primary function of expressing piety and encoding a belief in divine order. We no longer know how to read this code. It unites the physical with the metaphysical: according to Abbot Suger, building a church involved the transposition of the material into the spiritual. Artists of later ages, even until the present, have tried to achieve something analogous, but they have had no rules to guide them. Their attempts to forge materials into an expression of the ineffable therefore become highly personal visions, reflections of one individual’s spiritual world.
The theoretical principles governing the construction of the Gothic cathedrals were geometry and clarity. The structure of these buildings is dictated by proportion, by simple numerical relationships between the key dimensions. These mathematical relations were deemed to be expressions of perfection, a belief that stemmed from ancient Greek thought and for which some found endorsement in the Bible. So when we experience unity and order in Chartres Cathedral, it is the result of careful and rational planning, motivated not by aesthetics but by morality. The building expresses a conviction that the glory of God’s universe is expressed as a system of eternal order.
— From “Universe of Stone: A Biography of Chartres Cathedral,” by Philip Ball.