No doubt influenced by some post-Plotinian doctrines, Nebridius wonders what means the “demons” precisely use when they want to produce images in someone who is sleeping. He considers three possibilities, which are distributed as a main alternative, depending on whether the dream image is created by the soul of the dreamer (while being driven by the demons), or created by the demons, before being sent to the sleeper. But finally, Nebridius explains carefully why none of these hypotheses seems to him satisfactory. Augustine puts forward a fourth explanation, based on the interdependence of body and soul: in a nutshell, the demons may indirectly affect the dreamer’s soul by acting upon his body. Augustine’s reply is, as he says, intended only as the start of an investigation whose complexity disturb him. In fact, it is only the first step of the working-out of a problem that he will never solve satisfactorily. The study of the texts leads to a comparison with Porphyry, who seems to have faced the same problem with similar philosophical assumptions and similar puzzlements.