The unity of the Middle Ages as a historical period is quite artificial. In reality they fall into three entirely distinct cultural periods—the natural economy of the early Middle Ages; the courtly chivalry of the high Middle Ages; and the urban bourgeois culture of the late Middle Ages. At any rate, the divisions between these three epochs go deeper than those which mark the beginning and the end of the Middle Ages as a whole.

The Social History of Art, Volume I. From Prehistoric Times to the Middle Ages, 1999 - Chapter IV. The Middle Ages