Virgil, Georgics 4: Persephone imposes the condition only in a parenthetical aside, namque hanc dederat Proserpina legem; at the edge of daylight a sudden madness, subita dementia, seizes the incautious lover, and he looks back; Eurydice's farewell asks the story's question, quis tantus furor, what madness destroyed us both. Ovid, Metamorphoses 10, makes the law explicit, ne flectat retro sua lumina, and supplies the motive Virgil withheld: metuens ne deficeret, fearing she was failing him, avidusque videndi, eager for the sight of her. And Ovid adds the coda Virgil denies: reunited in Elysium, iam tuto respicit Orpheus, at last he looks back at her, safely (Met. 11.66).
The opera-and-guitar version: the greatest musician alive fails the simplest possible test. The popular moral is trust: she was behind you all along, why couldn't you believe it. Cocteau films it, Rilke rewrites it, and every band that ever titled a song Don't Look Back files its claim downstream of Virgil's parenthesis.
The poet's-dilemma reading, developed in scholarship on Virgil's poetics (Gale, Poetry and the Backward Glance in Virgil's Georgics and Aeneid): the look back is not a failure of nerve but the act of art itself. The poet cannot confirm the song is working without turning to see; Orpheus must verify the miracle and thereby kill it. On this reading he did not lose Eurydice, he traded her for the story, and every artist who has told the tale since has repeated the trade. Virgil built the recursion deliberately: a poem about poetry, ending where its own making began.
Did he look back because he doubted her or because he loved her, and does the text allow the question?