Judges, and tabloid newspapers, have a habit of saying that violent criminals ‘behaved like animals’. After all those years of David Attenborough they ought to know better: very few other species behave as badly as humans. But they are invoking the ancient belief that animals have neither reason nor justice. Consequently, ‘animal passion’ is fierce and overpowering. Animals are beasts. They are savage and unsocial. They cannot control their passions by reason and are unrestrained by respect for others, by a sense of fairness or by social order.

The Roman empress Eusebia, wife of the Christian emperor Constantius II (A.D. 337–361), owes what fame she enjoys amongst historians to her role in the life of Julian the Apostate, the last pagan emperor (361–363); in the years 354–355 the empress emerged as the saviour and advocate of her (still in the closet) pagan in-law. However in this article I wish to focus exclusively on the treatment of the empress Eusebia in the history of Ammianus Marcellinus, the great historian of Late Antiquity and himself a devotee of Julian. This treatment merits attention not just for the undoubtedly interesting fact that Eusebia is one of the few female characters to surface in the history, but also because in the history the empress is cast in seemingly different personae. She is the kind and beautiful empress, akin to the image Julian himself created of her in his Speech of Thanks to Eusebia, written in commemoration of his salvation at her hands; but she is also a devious player, as we find too in Zosimus' treatment of the empress, though in Ammianus her deviousness is malevolent. It is this contradictory characterization aspect of the treatment that I wish to explore here. For whilst general attempts have been made to reconcile these images in the context of political history, the repercussions for our understanding of Ammianus have been little explored. How can the historian let these contrasting images co-exist? Has he merely assembled his history clumsily, or is there a more rewarding explanation for the impression of the empress's split personality?