The five books of Artemidorus of Ephesus’ Oneirocritica (c. second century CE) constitute the largest collection of divinatory dream-interpretations to survive from Graeco-Roman antiquity. This article examines Artemidorus’ contribution to longstanding medico-philosophical debates over the ontological and epistemic character of such dreams. As with wider Mediterranean traditions concerning premonitory dreams, Greeks and Romans popularly understood them as phenomena with origins exterior to the dreamer (e.g. a visitation of a god). Presocratic and Hippocratic thinkers, however, initiated an effort to bring at least some dream events within the body as interior processes of a physical soul, a bodily turn which ultimately engendered a split in thinking about the possibility and scope of prophetic and diagnostic dreams (e.g. Hippocrates Regimen 4, Aristotle On Prophecy in Dreams, Rufus of Ephesus, and Galen). Here, I examine the way that Artemidorus meant the Oneirocritica to address seriously questions about the origins of dreams and the powers of empirical science in two, related ways. We will see that his defense of oneirocriticism as a techne unfolds along two main axes. First, Artemidorus constructs a necessary veneer of scientific credibility by mobilizing the tropes and organizational strategies characteristic of contemporary technical literature. Secondly, he anchors that authority upon a materialist account of prophetic dreams as “semiotic movements of the soul.” As Price, S. 1986. “The Future of Dreams: From Freud to Artemidorus.” Past & Present 113: 3–36 and Harris-McCoy, D. 2012. Artemidorus’ Oneirocritica: Text, Translation, and Commentary . Oxford: Oxford University Press observed, this explicit commitment to an empirical method of dream-interpretation resembles that of the Hellenistic medical ‘sect’ known as ‘Empiricists,’ who eschewed interest in theoretical etiology in favor of observation, comparison, and compilation of symptoms and outcomes. Pushing beyond similarity, I argue that Artemidorus not only deployed this method to defend empirically grounded accounts of dream-divination (cf. Cicero De div. 1), but endeavored to show that such a scientific method can be used to stabilize and corral productively the potentially infinite number of interpretations any dream-sign might bear without recourse to an underlying, causal theory.