The twenty-two volumes of the last edition of the collected works of Galen occupy a smaller place in the affections of classical scholars than on the library shelf. The reputation of Galen as a great physician is sufficient to ensure a respectful neglect of his writings that is as undeserved as it is effective and that hampers a reassessment of his aims and abilities. Even though recent scholarship has done much to show him as a recognizable product of and participant in the Second Sophistic movement with strong claims to be regarded as a universal scholar, Wilamowitz's dismissal of him as a ‘Seichbeutel’ still remains a typical verdict. Not that the epithet was entirely unjust, for Galen's prolixity, his frequent polemics, his repetitions and recapitulations, and the inelegance of much of his prose style are all obstacles to his readers, few of whom have had the stamina and enthusiasm, to say nothing of the linguistic competence, to read all of his works preserved in Greek, Latin, Arabic and Syriac. Thus it is not surprising that the finest contemporary account of society in Rome during the reign of Marcus Aurelius, which lies buried deep in the fourteenth volume of Kuhn's edition, has only rarely been utilized in a discussion of that period. Πɛρὶ τΞῦ πρΞγινώσκɛιν, ‘On prognosis’, whose title, given by Galen himself, disguises the variety of its contents, has remained unread and its absence from the three volumes of the Scripta Minora Galeni and, as yet, from the Corpus Medicorum Graecorum has placed it beyond the ken of most historians. It is my intention in this paper to present a description of this treatise and a discussion of its position within literary genres as a contribution to the history of autobiography.