In the medical practice of Asclepian dream incubation, dreams offered a conduit through which the divine power of the healing god could be visited upon an ailing suppliant. This practice was enough of a part of everyday life in fifth-century Athens that it achieved the dubious honor of an extended parody in Aristophanes’ Plutus. An extensive inscriptional record suggests that it continued to flourish for many centuries. But there was another type of dream employed in ancient Greek and Roman medical practice, with a much scanter trail of evidence. These dreams had endogenous, physiological origins and provided information about the internal disposition of the body not by divine intervention, but by some manner of inward perception on the part of the patient. With the rising interest in observational methodology in the fith century, opsis, and ideally autopsy, became the basis on which scientific knowledge was produced and elaborated. Taboos against physically opening the human body, in life as well as in death, prevented physicians from directly observing their patients’ interiors. The visions of dreams, then, could potentially provide doctors with a uniquely valuable diagnostic tool: genuine access to the observation of a body's internal condition, albeit in a strange, mediated form.