The little treatise On Diagnosis from Dreams, or Peri tês enypniôn diagnôsis, appears as pp. 832-835 of Volume VI in Karl-Gottlob Kühn's edition of Galen. It presents a brief outline of the reasons for regarding dreams as a useful tool in diagnosis. Despite its brevity, On Diagnosis from Dreams presents a coherent theory of medical dreams which is consistent with Galen's outlook.
There are grounds to believe that On Diagnosis from Dreams is not in fact an independent work by Galen. It begins with kai, as though in mid-thought. Its wording echoes a passage in Galen's Commentary on Hippocrates' Epidemics I, where Galen is discussing Epidemics I.23. In that commentary Galen says, "Moreover, I have expounded also concerning dreams, both others and the kind that indicate some disposition of the body, as has been described also in the On Regimen in Health [i.e. Hippocrates Regimen IV or Dreams]." He then continues, in language almost identical to that of the first paragraph of On Diagnosis from Dreams, "For someone dreaming conflagrations . . .."
At On the Natural Faculties I.12 (Kühn II.29) Galen mentions another work in which he set forth his ideas on dreams. This cannot be On Diagnosis from Dreams, since it seems to have treated divination in general as well as the doctrines of Asclepiades. Neither Galen's On Regimen in Health nor the work cited in On the Natural Faculties is extant. If On Diagnosis from Dreams is not by Galen, it may be an attempt by some later medical writer to reconstruct his doctrine on the basis of the hints in his commentary on the Epidemics, or perhaps from the lost work On Regimen in Health.
A third possibility remains. At On Treatment by Venesection 2 (Kühn XI.254) Galen acknowledges that he could prepare short treatises on specific topics for readers who found his major works too much to handle. On Diagnosis from Dreams may well be such an excerpt, prepared not by an anonymous redactor but by Galen himself.
On Diagnosis from Dreams has come down to us as an independent treatise in 12 manuscripts. The oldest, Vaticanus Lat. 1063, dates to the 13th Century; the most modern, Oxoniensis misc. 130, to the 17th. Giulio Guidorizzi grouped these manuscripts into three families:
Vaticanus Lat. 1063
Ambrosianus A 156 sup.
Vossianus (4o) 45
Oxoniensis misc. 130
Marcianus app. cl. V,5
(A 13th manuscript, Vindobonensis med. 30, although listed by Diels in his 1905 census of medical manuscripts, appears not to contain On Diagnosis from Dreams.) In addition, 19 manuscripts transmit an anonymous Latin version of On Diagnosis from Dreams. Fahd 1966, 338, reports an Arabic translation.
H. Diels, "Die Handschriften der antiken Ärzte, I," Abhandlungen Preuss. Akad. Wiss. 1905.
R. J. Durling, "Corrigenda and Addenda to Diels' Galenica," Traditio 23(1967), 373-81.
T. Fahd, La Divination Arabe: Études religieuses, sociologiques et folkloriques sur le milieu natif de l'Islam (Leiden, 1966).
Giulio Guidorizzi, "L'opusculo di Galeno De Dignotione ex insomniis," Bolletino del comitato per la preparazione dell'edizione nazionale dei classici greci e latini n.s. 21(1973), 81-105.
Steven M. Oberhelman, "Galen, On Diagnosis from Dreams," Journal of the History of Medicine and Allied Sciences 38(1983), 36-47.