Didascalicon, Book II: the sciences, their parts and their discipline
Chapter XVI. Definition of the quadrivium
Arithmetic, then, is the science of numbers; music is the division of sounds and the variety of voices. Otherwise, music or harmony is the concord of many unlike things brought into one. Geometry is the discipline of immovable magnitude and the contemplative description of forms, by which the limits of each thing are wont to be declared. Otherwise, geometry is the fountain of the senses and the origin of expressions. Astronomy is the discipline investigating the spaces, motions and returns of the heavenly bodies at fixed times.