Against Celsus 4.91

Moreover, if omen-birds fight omen-birds, and if, as Celsus says, the prophetic birds and the other irrational animals, having a divine nature and notions of the divine and foreknowledge of the future, foretold such things to others, then neither would Homer's sparrow have nested where a serpent was going to destroy her and her young, nor would the serpent in the same poet have failed to guard against being seized by the eagle. For Homer, admirable in poetry, says this about the first: "Then a great sign appeared: a serpent, blood-red on its back, terrible, whom the Olympian himself sent into the light, darting from beneath the altar, rushed to the plane tree. There were the young of a sparrow, tender nestlings, on the topmost bough, cowering beneath the leaves, eight of them, and the mother that bore the young was the ninth. There he devoured them as they cheeped piteously, while the mother fluttered round, lamenting her dear young; but he coiled himself and seized her by the wing as she shrieked. But when he had eaten the sparrow's young and herself, the god who had shown him made him conspicuous: the son of crooked-counselling Cronus turned him to stone. And we stood and marvelled at what had happened. So the dread portents of the gods came in among the hecatombs." And about the second: "For a bird came upon them as they were eager to cross, a high-flying eagle, skirting the host on the left, carrying in its talons a blood-red serpent, monstrous, alive and still writhing; and it had not yet forgotten the fight. For it struck the eagle that held it on the breast beside the neck, bending itself back; and the eagle, stung with pain, let it fall from him to the ground and dropped it in the midst of the throng, and itself, with a cry, flew off on the blasts of the wind. And the Trojans shuddered when they saw the gleaming snake lying in their midst, a portent of Zeus who bears the aegis." Was the eagle, then, prophetic, and the serpent, since augurs use this animal too, not prophetic? And since it is easy to refute the choice of one over the other, would not the claim that both are prophetic be refuted too? For would the serpent, if it were prophetic, not have guarded against suffering this from the eagle? And one could find countless other such examples showing that it is not the animals that have a prophetic soul in themselves; rather, according to the poet and most people, "the Olympian himself sent it into the light," and for a certain sign Apollo too uses a hawk as messenger; for "the falcon" is said to be "Apollo's swift messenger."