De institutione musica II.3: De differentiis quantitatis et quae cui sit disciplinae deputata.
All quantity, according to Pythagoras, is either continuous or discrete. The continuous is called magnitude, the discrete multitude. Their property is different and almost contrary. For multitude, beginning from a bounded quantity, grows and proceeds without limit, so that no end of growing is met; it is bounded toward the least, unbounded toward the greater, and its beginning is the unit, than which nothing is less. It grows through numbers and extends without limit, and no number sets a term to its growing. Magnitude, however, receives again a bounded quantity of its own measure, but decreases without limit. For if there is a line of a foot or of any other measure, it can be divided into two equal parts, and its half cut in half, and that half again into another half, so that there is never any end to the cutting of the magnitude. So magnitude, as regards the greater measure, is bounded, but becomes unbounded when it begins to decrease. Number, on the contrary, as regards the lesser measure is bounded, but begins to be unbounded when it increases. Although these things are thus unbounded, philosophy nevertheless treats of them as of bounded things, and found in unbounded things something determinate, to which it could rightly apply the keenness of its own speculation. For of magnitude some kinds are immovable, as the square or the triangle or the circle, others movable, as the sphere of the world and whatever is turned in it with a fixed swiftness. Of discrete quantity some are by themselves, as three or four or the other numbers, others relative to something, as the double, the triple, and others which arise from comparison. Geometry holds the speculation of immovable magnitude; astronomy pursues the science of the movable; arithmetic is the author of discrete quantity; and music is proved to have the skill of quantity related to something.