It is essential, when considering the roles and social position of the providers of healing in the ancient world, to be careful about assuming that they formed a coherent group whose status, reputation and ideology can be clearly determined.1 It is true that some ancient authors talked about a ‘profession of medicine’ and sought to lay down universal standards of behaviour, but the context in which they made these demands suggests that they were frequently disregarded, not least because many of the institutions that today support the notion of a medical profession were entirely absent.2 Similarly, even the broadest of definitions of a doctor, as a person who offered to treat illness for money, does not allow for the fact that he or she might be carrying on other profitable activities at the same time, or might have switched to medicine from a totally different occupation.3 Even distinguishing terms such as ‘high’ and ‘low’, ‘formal’ and ‘informal’ medicine cannot entirely capture a historical situation in which the predominant feature is the fluidity of all defining boundaries. Indeed, rather than employ a succession of dichotomies demarcating good healers from bad, professional from amateur, religious from secular, in an attempt to define what an ancient doctor was and did, one might more appropriately adopt the metaphor of a series of overlapping spectra, on which can be mapped the great diversity of data concerning the wealth, status, education, ideology and methods of the practitioners of healing in Antiquity, particularly in the first three centuries of the Christian era. That there were lines of demarcation between those who were called, or who called themselves, ‘doctors’, iatroi or medici, and those who were not, is beyond doubt. But where those lines were drawn was a matter of individual choice and of individual context, as we shall see. Galen’s view of who should be a doctor, and what he or she ought to be able to do, differed substantially from that of Thessalus of Tralles, and the perspective of both of these metropolitan practitioners would in turn differ from that of a patient or a doctor in Britain, the Nile Valley or the mountains of central Asia Minor. Besides, the title of doctor in no way guaranteed competence.4 As the Alexandrian Jew Philo put it around AD 50:

In medicine there are those who know almost everything there is to know about treating affections, diseases and ailments, yet who can give no account of them, whether accurate or even plausible. On the other hand there are those who are marvellous expositors of the signs, causes and therapies that make up the art of medicine, but who are hopeless at treating the sick and contribute not a jot to their recovery.5