The ancient mystery cults practiced by the Greeks and their neighbors were derived from the prehistoric use of caves as places to induce altered states of consciousness. The visions obtained in these rituals were expressed in both cave paintings and in the stories of the ancient playwrights. Ancient thought was shaped profoundly by the ecstatic experiences of Euripides and others, whose own experiences in visionary states were externalized in the form of their poetry and plays. Plato’s allegory of the cave represents a rejection of the “irrational” Dionysian way of knowing, and thus a denial of a long-enduring source of human knowledge, and a preference for a “rational” Apollonian way of knowing. Our modern attitudes continue to reflect this Platonic prejudice. Keywords: Euripides; Plato’s allegory; Dionysian ecstasy; Apollonian reason; caves