From Bernard Berenson’s Piero Della Francesca or: The Ineloquent in Art (pages 3-5):
Piero della Francesca seems to have been opposed to the manifestation of feeling, and ready to go to any length to avoid it. He hesitated to represent the reaction which even an inanimate object would have when subjected to force, the rebound of a log, for instance, when struck by an axe.
In the Borgo San Sepolcro fresco the Resurrected Christ, a sturdy stevedore like the Baptist in the early polyptych of the same little town, or the Christ in the London Baptism, looks straight ahead of him, dazed and as if waking from a refreshing sleep. It would take great imaginative power to discover in the two other figures just mentioned the faintest correspondence between looks and function. No Holy Spirit could penetrate the head of the grim athlete standing in mid-stream of Jordan. Three Angels, the comeliest figures Piero ever painted, stand by, but it is not certain that any of them is participating.
One is almost compelled to conclude that Piero was not interested in human beings as living animals, sentient and acting. For him they were existences in three dimensions whom perchance he would have gladly exchanged for pillars and arches, capitals, entablatures, and facets of walls.