in pairs preserves a decorum in spite of the vigorous movement pictured on the doors.  These doors open and shut:  they were meant to do so, especially to shut.  Ghiberti’s second pair of doors for the Baptistery do not shut:  they are closed, but they do not give the sense of shutting anything in or keeping anything out.  They are more like windows than doors.  They give no impression of defence or resistance:  they are doors in nothing but name, and the chance that they hang on hinges.  Were it merely a contest between Ghiberti and Donatello as to which sculptor were the more skilled constructor of doors, further comment would be unprofitable; but it raises the wider question of the laws and limitations of bas-relief—­the application to sculpture of the principles of painting; in short, the broad line of demarcation between two different arts.  Michael Angelo probably realised the unity of the arts better than Donatello, but Donatello knew enough to treat sculpture with due respect:  he valued it too highly to confuse the issue by pictorial embellishments.  It is no question of a convention, still less of a canon.  But there are inherent boundaries between the two arts; and where the boundaries are overstepped, one or the other art must lose some of its essential quality and charm.  Donatello’s reliefs at Padua are crowded:  Ghiberti’s (on the second gates) are overcrowded.  The difference in degree produces a difference in principle.  If Ghiberti had made pictures instead of reliefs, the atmosphere would keep the objects in their right places, while differences of colour would give distinction to certain parts and the chief figures would still predominate.  In other reliefs Ghiberti lavished so much care on landscape and architecture that the figures become of secondary importance:  on one relief a tree casts its shadow on a cloud.[172] Ghiberti, in fact, with all his plastic elegance, with a grace, suavity and sense of beauty which Donatello never approached, was a painter at heart. “L’animo mio alla pittura era in grande parte volto,” he says in his Commentary,[173] and the faults of his sculpture are due to this versatility.  Donatello only used his pictorial knowledge to perfect form and feature; and, complex as his architectural backgrounds often are, they never suggest experiments in perspective, and they never detract from the primacy of the people and the incident.  Michael Angelo was under no illusion on this point:  he never confused painting and sculpture.  Yet he said Ghiberti’s gates would be worthy portals of paradise. “Ce n’est pas la seul sottise qu’on lui fasse dire,” drily remarked the Chevalier des Brosses;[174] and, curiously enough, about the time that Michael Angelo made his famous Judgment, an amateur of the day made a much shrewder criticism, long since forgotten, that the doors would be adequate to stand at the gates of Purgatory:—­“sarebbon bastanti a stare alle porte del Purgatorio."[175] The ambiguity